Art. XL.—Terminology for Beak and Foraminal Development in Brachiopoda.
[Publication authorized by the Publication Committee under Regulation 5, (a), (2); issued separately, 26th August.]
The previous communication made by me on this subject, entitled “Terminology for Foraminal Development in Terebratuloids (Brachiopoda),” which appeared in the Transactions of the New Zealand Institute, vol. 48, 1916, pp. 130–32, seems to have provided terms which are found to be useful and convenient. It has also received commendatory notice in Revue critique de Paléozoologie, 1918, pp. 37–39, though the writer of the review has fallen into an error which may be misleading. For these reasons I am tempted to pursue the subject somewhat further.
The error committed by the writer in question is this: He says (p. 38), “M. Buckman a indiqué graphiquement la position du foramen circulaire par rapport à la ligne de séparation des valves.” This is a complete misunderstanding of the little diagram in my paper. The position of the foramen is not indicated with regard to “the line of separation of the valves,” but in regard to the line of the beak-ridges (la ligne des carénes laterales du crochet)—quite a different matter. The line of the beak-ridges is the convenient and necessary datum-line for the purpose, and it is this line which is represented by the horizontal lines in my diagram. This line of the beak-ridges separates a more or less defined area, the pseudo-area or cardinal area of the ventral valve.
There is another little matter for notice in this quotation. The circular foramen is not a necessity in regard to these terms. That the foramen was indicated by a circle was only a matter for convenience in printing. The position of the foramen is one observation; the shape of the foramen is another—quite distinct; and the condition of the foramen is a third. These must be kept separate, for all sorts of combinations are possible.
The shape and some of the developmental phases of the foramen come under the terms “trigonal,” “subtrigonal,” “elliptical,” “circular,” and so forth—see my memoir, “The Brachiopoda of the Namyau Beds,” Palaeontologia Indica, n.s., vol. 3, part 2, 1917 (1918); but other terms, like “obtrigonal,”* “oblong,” “trapezoid,” &c., will be required. The condition of the foramen I have described by such terms as “telate,” “marginate,” “auriculate,” “attrite,” “labiate,” “renovate.” Some of these conditions are found only with the hypothyrid position, others belong to the mesothyrid in the main or to the epithyrid. Particulars are given in the work above quoted under the headings of “Rhynchonellidae” and “Terebratulidae,” but I am unable to cite the page-numbers, as the only copy which has yet reached me is at the binder's.
Returning now to the position of the foramen, it may be desirable to have terms by which to indicate its position in regard to the line of the
[Footnote] * With apex pointing anteriorly.
valve-junction; but before proposing these it is advisable to consider the general course of development which has led to the specialized foramen of the Telotremata: it is to such foramens that the various terms hitherto proposed apply.
(1.) The pedicle-opening or delthyrium is unmodified, and is therefore in area coextensive with what may be called an opening or foramen.
(2.) The pedicle-opening is modified by the growth of various plates, so that the area of the foramen is less than the area of the delthyrium.
(3.) The pedicle-opening moves, as new test is deposited, away from the apex towards the posterior margin, and the abandoned track is closed by a plate called a “listrium” (Discinacea).
(4.) The pedicle-opening moves in the opposite direction, out of the delthyrial area, by the pedicle gradually absorbing the apex. This is the development found in submesothyrid to epithyrid Telotremata.
Now, as the modifications of the foramen take the form of constricting the area or a shifting of position, it may be seen that the terms “Neotremata,” and so forth, which suggest something bored out, are not altogether appropriate. Only in the last case, where old test is removed, is there anything like the process of boring. Closing up a space so as to reduce a hole is something quite distinct from boring out. However, that point is not for consideration now.* What has to be dealt with is what may be called the character of the foramen.
When the foramen and the delthyrium correspond the foramen might be said to be delthyrid in character. When the foramen is less in area than the delthyrium, by the growth of various plates which modify the delthyrial opening, then the term constricted foramen seems appropriate. When the foramen is shifting or has shifted its position more or less out of the delthyrial area, then the foramen is in its character migrant or migrate, as the case might be. The mesothyrid Telotremata have migrant foramens, and the epithyrid, migrate. When the foramen moves as new territory is developed it may well be called emigrant, and when it goes to take up old territory—the Terebratuloids—it could quite well be said to be immigrant.
Using the term “foramen,” therefore, in the above general sense as the pedicle-opening, which in character may be unmodified, modified, or migratory, it is necessary to define its position with regard to the line of the valve-junction. It is situated in the ventral valve, and the obvious term would be
Gastrothyrid (γαστν´ρ, belly). All the position-terms, “hypothyrid,” “mesothyrid,” “epithyrid,” relate to and are more precise locations of the foramen in gastrothyrid Brachiopoda.
If the foramen were situated in the dorsal valve, then the term would be Notothyrid (νω∘τos, back). I know no such case; but according to the statement in the review above noticed the hypothyrid position would be notothyrid. It is, however, gastrothyrid, and hypothyrid because under the apex.
Then there are cases in which the foramen is situated at the junction of the two valves, part of the foramen being in each valve. For this it is not so easy to obtain a suitable term, but I would suggest
[Footnote] * It might be suggested, however, that “Eotremata” would be preferable to “Atremata,” understanding τρν∘μα in the sense of opening.
Symbolothyrid (from σvμβoλν´, the part that meets, the joining, the end).* Symbolothyrid is the foraminal position in many Atremata, in early forms of the Telotremata, and is also found in Platystrophia. “Coenothyrid” (from κoιvós, shared in common) also suggests itself, but would have to be rejected to prevent confusion with Coenothyris. It would be inadvisable to have to say that Coenothyris was not coenothyrid but gastrothyrid. In regard to the symbolothyrid foramen there is the possibility of its greater portion being in one valve and the lesser part in the other. Such cases could be described as symbolothyrid inclining to gastrothyrid or to notothyrid, as the case might be.
Some special developments in the structure and conditions of the foramen now seem to merit distinctive terms. The foramen is in the form of a tube extending into the valve: the shell is therefore
Siphonothyrid (σι´Φωv, a tube), as in the Siphonotretidae and in Syringothyris. A siphonothyrid shell is to be distinguished from one in which the whole beak projects forwards as a little tube well separated from the dorsal umbo. This is a case of a tubular beak, and is found, for instance, in Terebratula wrighti Davidson, of the Bajocian, to which I have given the generic name Tubithyris.† A greater development still of the tubular beak is seen in Lyra meadi (Terebrirostra), where the beak is produced into a long pipe. This may be termed a fistulate rostrum or beak (fistula, a pipe). It is the whole beak which is under observation, not merely the foramen; that, I expect, is epithyrid attrite, but I have no example of this interesting and rare shell.
Cryptothyrid (κρvπτós, hidden) is a term now suggested to denote a foramen hidden or concealed by the beak, as in the case of Athyris, which originally obtained its name (α, not; θvρι´s, foramen [window]) because it was supposed that it lacked a foramen. It is quite possible that the pedicle was almost functionless.
In Productus the pedicle certainly was functionless, the foramen being more or less sealed up, the animal having no need to use the pedicle, as it could anchor itself by its long spines. For this condition the term
Clistothyrid (κλ∊ιστós, shut up) seems suitable. I would prefer to write it “cleistothyrid,” to signify that the first i is to be pronounced long, but such is not the rule that has been laid down.
I am under the impression that a minute Rhynchonella of the Upper Inferior Oolite to which I have given the name Nannirhynchia‡ is clisto-thyrid. The foramen seems to be closed, but observation in regard to this feature of so small a shell is difficult. This tiny shell has a hirsute test, sufficient, perhaps, to anchor so small an object.
A further development of the clistothyrid condition is when the foramen is not only closed with calcareous matter, but the shell is cemented down to the object of attachment. For this the term
Calcithyrid (calx, lime) seems obvious. Objections to it as a barbarism are sure to be made; but they neglect the fact that a word is for use first
[Footnote] * I have already proposed for this position of the foramen the term amphithyrid (Brachiopoda, Australian Antarctic Expedition, 1911–14, Scientific Reports, ser. C, vol. 4, pt. 3, p. 20, 1918). Mr. Buckman, who has seen the manuscript of my paper, but is apparently unaware that it is published, writes that the name is not as happy as it might be, as the Greek αμΦvρos means “with door or both sides, a double entrance,” suggesting a foramen in each valve. As there is no definite law of priority in such terms, it is preferable to use symbolothyrid.—J. Allan Thomson, 25/6/19.
[Footnote] † Records Geol. Surv. India, vol. 45, p. 78, 1915.
[Footnote] ‡ Gen. Jur. Brachiopoda, p. 2, 1914.
and for ornament afterwards. It is sure to be said that one should take the Greek word of similar meaning—χα´λι`ζ—and write “chalicothyrid”; but I am not in favour of this far more cumbrous and less obvious word. The Craniidae, Thecidiidae, and Richthofenidae are some of the cemented shells and are therefore calcithyrid. If it is urged that the development in the last-named is so much greater that it deserves distinction, then I make it a present of “chalicothyrid”; it does seem rather to suit that family.
The above terms would seem to provide a ready means of describing some important aspects of beak and foraminal development. But there is a further case to be considered—the position of the foramen in shells which have considerable development of the cardinal area or its homologue the false cardinal area. Two terms for what is really the same feature are very unsatisfactory. There is some objection to calling this feature simply “the area,” for that term is required for general use; but it might be termed the interarea, as being the area lying between the apex and the posterior line of valve-junction—the cardinal margin when there is a hinge, but at any rate the posterior margin When there is lateral compression posteriorly by which areas are formed each side of the beak—shown, for instance, so well in Rhynchonella plicatella—then the term “areola,” which is in use, seems unsuitable; for it is larger, not, as its name suggests, smaller, than the cardinal area. The term planarea seems advisable. There would thus be the interarea and the planareas; but the presence of the one would, I think, prevent the other appearing in the same shell: at least, the former is merged in the planareas.
With great development of the interarea the term “hypothyrid” for the position of a foramen in the interarea might not be sufficiently precise. It could be modified as “apically,” “medianly,” or “marginally” hypothyrid, as the case might be.
Mention of the apical position suggests that it is desirable to distinguish the rostrate shells which keep the apex and those which lose it: the former, like the Rhynchonellidae, would be shells with rostrum apicate; the latter, like Terebratuloids, with rostrum truncate. It has been usual to speak of the foramen as being truncate; but this is mainly from a consideration of the attrite foramen of the Terebratulids. But the attrite condition is expressive of the wearing-away of the tela—the points of the beak-ridges; and so it seems advisable to apply the term “truncate” to the rostrum as soon as the apex is excavated by the pedicle. The angle and the degree of such truncation, especially when combined with attrition, may vary very considerably—obviously through nearly 180 degrees. To meet such cases “subtruncate” or “undercut,” “vertically,” “obliquely,” and “horizontally” truncate might suffice for general purposes. Words compounded of in and re are to be avoided; for properly the latter should apply to what goes backwards—that is, towards the dorsal valve Too often the former has been used for such cases—the legacy of old teaching Peccavi!
One other point is for consideration. When the pedicle has eaten through the apex and has attained the epithyrid position, or even before, it seems to have a tendency to return—to move back towards the dorsal valve again: it may be said to be in character remigrant. In such case it produces a labiate foramen with some recutting of the dorsal wall of the foramen. In such recutting points might be produced which resemble tela but are not true ends of beak-ridges, only projecting edges of foraminal
rim: if so, they should be termed pseudotela, and the foramen would be pseudotelate. Further investigation of this matter is required, and perhaps may best be undertaken by those who have the opportunity to obtain and study fresh Recent specimens. With such material at hand notice might be taken of the part played by the pedicle in depositing test in the foramen. In the inside of the labium of fossil Terebratulids with labiate foramen may be found a tongue-shaped layer of test which one supposes to have been deposited by the pedicle: if so, this should merit distinction as the pedicle-plate, to be looked for in connection with a remigrant foramen. Then the question comes whether similar secretion by the pedicle has played any part in the fusing of the deltidial plates. To the fused deltidial plates as shown by Terebratulids I have given, in the memoir already mentioned, the name of symphytium, from the likeness to the closing-up of a wound. In some well-preserved epithyrid Terebratulids the middle area of the symphytium seems to be distinct, with very considerable resemblance to the pedicle-plate. And in some shells the symphytium seems to be composed entirely of this middle piece. Now, there arises the suspicion that this middle piece has been laid down at first in between, and later on the foraminal edge of, the coalescing deltidial plates. For if there was simple coalescence of the deltidial plates, growing together from the edges of the delthyrium, as they certainly do in hypothyrid Rhynchonellids with concrete deltidial plates, then the lines of growth should run longitudinally, more or less parallel with the diverging edges of the delthyrium. But in the piece under consideration the lines of growth appear as transverse rugosities—the transverse direction being what would be expected from deposition by the pedicle, and the rugosities being connected with the swelling and attenuation of the pedicle under muscular stimulus—connected possibly with periodical variations of marine conditions.
Now, if such arguments be correct, the symphytium—the final closing of the deltidial plates—in epithyrid shells is to be distinguished altogether from the conjunct or concrete deltidial plates of hypothyrid shells. One should find, perhaps, in mesothyrid shells concrete deltidial plates united near the cardinal margin, but separated medianly (that is, the part nearer the apex), and, lying in this separation, the beginning of the symphytium with its transverse rugosities; and in epithyrid shells, a later development, perhaps only the symphytium, margined possibly by a relic of the deltidial plates.
To observe these details in fossil shells means much excavation, with results not always satisfactory, on account of defective preservation. One may hope that those who have the opportunity to examine Recent epithyrid shells may be able to supply further details.