Art. XIII—Terminology for Foraminal Development in Terebratuloids (Brachiopoda).
The excellent work which is being done by Dr. J. Allan Thomson among the Brachiopoda of New Zealand, and his thorough grasp of modern principles of palaeontology, are abundantly proved in the pamphlets which he has recently published. They are very welcome, they make a genuine and satisfactory advance in knowledge.
There is, however, a slight ambiguity in a certain phrase used by the author, due to a lack of technical terms, and as I have already proposed the necessary terms, which have been in type now for about a year, in a publication on Brachiopods to be issued by the Geological Survey of India, it seems desirable to mention them, so that they can be utilized, because, for various reasons, it may yet be some time before my larger treatise can he published, and it seem advisable to avoid the possible complexity of two sets of terms for the same features.
In his paper “Brachiopod Genera” Dr. Thomson says, “All the known species of Bouchardia . possess similar and rather unusual beak characters. there are sharp beak-ridges uniting in front of the foramen, which its thus behind the apex”* Here is the ambiguity referred to, contained in the words “in front” and “behind”; and perhaps I notice it the more readily because I have myself admittedly stumbled in similar manner on more than one occasion. Now, the ambiguity is this: The above words, “in front” and “behind,” are exactly contrary to the
[Footnote] * Trans. N.Z. Inst., vol. 47, 1915, p. 397.
use of the terms “dorsal” and “ventral valves” in Brachiopods. As it happens, these shells are so generally examined and depicted in dorsal view that one unconsciously comes to think of the dorsal valve as the front. Really it is the behind valve, and so the beak-ridges of Bouchardia unite behind (dorsally of) the foramen, which is thus in front (ventrally of) the apex.
The terms which I have proposed are designed to meet this and similar cases—that is, to express directly the position of the foramen in regard to the beak-ridges; for there is development in its position—the pedicle. shifts from the pseudo-area, cuts through the beak-ridges, destroying the apex, and takes up a position in the ventral umbo. The terms are modications of Phillips's old generic names Hypothyris, Epithyris, bestowed originally to mark just such differences in the position of the pedicle. Thus the foramen is—.
Hypothyrid when it is in the pseudo-area, and the apex is intact; this is a usual condition among Rhynchonellacea, but is rare among Terebratulacea:
Submesothyrid when the apex has been absorbed, yet the foramen lies mainly in the pseudo-area but partly in the ventral umbo; this is a frequent condition in Terebratulina:
Mesothyrid when the foramen lies about equally each side of the beak-ridges, a usual condition in Mesozoic Dallininae:
Permesothyrid when little of the foramen lies on the pesudo-area but the main of it is in the ventral umbo, a condition also found in Mesozoic Dallininae and in Terebratulidae:
Epithyrid when the foramen lies, wholly in the ventral umbo and the line of the beak-ridges passes dorsally of it; this is a usual condition in Terebratulidae.
These are five sequent stages in the shifting of the pedicle ventralwards, of which (1) is the earliest and (5) the latest. The beak-ridges make a datum-line for observation. The Dallininae in general occupy an intermediate position between the bulk of Rhynchonellidae and the bulk of Terebratulidae—at any rate, so far as Mesozoic species are concerned.
There are, however, still other features connected with the foraminal development which require technical terms. As the pedicle absorbs the apex, eating through a line of strong beak-ridges in shifting its position, the ends of the beak-ridges remain like little darts projecting on each side of the foraminal opening—foramen telate. At a later stage these darts become worn off—foramen attrite, well seen in Magellania. Still later the opening is finished off with a deposit of test, a kind of rim—foramen marginate; and in further development a lip is projected over the dorsal umbo—foramen labiate. This lip seems to indicate that the pedicle has more than attained its farthest limit ventralwards, and that it is now beginning to return on its path to take up a position more dorsalwards.
These modifications of the condition of the pedicle-opening do not keep step with those other modifications of its position. The consequence is a series of varied combinations, which may be of considerable utility in the diagnoses of genera. They also enable the beaks of species to be placed in developmental position in regard to one another, and, if there is incongruity, suspicion may usefully be aroused as to whether a species is rightly placed, suggesting investigation of its internal details.
For instance, the beak of Bouchardia is, in regard to position of foramen, just attaining to the epithyrid stage—perhaps something of the permeso-
-thyrid stage may be detected, but the condition of the foramen is telate, for the ends of the ridges are not worn off. But in Rhizothyris, which Dr. Thomson, has, on internal characters, separated from Bouchardia, the beak becomes an external tell-tale. According to his figure of R. rhizoida,* the foramen is mesothyrid attrite. Therefore, in position the foramen of Bouchardia is more advanced than that of Rhizothyris, but in condition it is less advanced. These features indicate that these forms are not lineal relatives, but are collaterals. The precedent beak-stages which may be expected in some ancestor of both genera would be mesothyrid telate; but the divergence may have begun earlier.†
The foraminal positions may be diagrammatically illustrated by rules and circles in the following mariner, where the rule stands for the line of beak-ridges and the circle for the foramen:—
These notes do not exhaust the subject, even for Terebratuloids. A fuller discussion of these and of other development phases is in type for the memoir in the “Palaeontologia Indica.” It is hoped, however, that these preliminary remarks may be of service until that work appears.
[Footnote] * Trans. N.Z. Inst., vol. 47, 1915, p. 398, Fig. 5a
[Footnote] † According to specimens of Rhizothyris rhizoida very kindly sent by Dr. Thomson, the foramen is permesothyrid attrite. This slightly modifies the above statement, but does not invalidate the argument.—2nd March, 1916.