Art. LVII.—On the Genus Rhynchonella.
[Read before the Wellingtou Philosophical Society 10th October, 1880.]
It is by the permission of the Director of the Geological Survey that I have the pleasure of placing this paper before the Society.
Of the mollusca Rhynchonella, although it is represented by but two living species, is, if the fossil species be taken into consideration, numerically the most important genus belonging to the Brachiopoda.
In Woodward's “Manual of the Mollusca,” the genus is said to include 332 fossil species; some 60 species obtained in New Zealand have to be added, thus bringing the total to something like 400 species.
At the present time R. nigricans alone survives in the southern seas, and is found on the New Zealand coast.
This species is the only one found in our upper and middle tertiary strata. In the upper Eocene rocks of New Zealand, represented by the Mount Brown and Hutchinson quarry beds, another form appears, but this with its close ally R. nigricans is the only species yet known from our tertiary strata.
From our upper secondary rocks two more species are added to the list, Rhynchonella Squamosa, Hutton, and a species not yet described, which is found in cretaceous rocks of the East Coast of Auckland.
Fossils belonging to this genus abound in the cretaceous rocks of England, and most other countries in the Northern Hemisphere. About twenty species have been described from the English deposits of this age. The contrasting scarcity of Rhynchonellidæ in the New Zealand cretaceous rocks appears something like evidence in favour of the opinion, that the decline of the genus commenced earlier in the southern than in the northern hemisphere. Whether or not this speculation has the importance which I would thus attach to it, curiously enough, it finds partial confirmation in the fact that the molluscous fauna of our cretaceous seas assumed a tertiary type of character considerably prior to the period of such change in the Northern Hemisphere, within the European area.
In the middle and lower-secondary rocks of this country Rhynchonella is represented by a great increase of the number of species, and in many strata the rocks are crowded with such shells. With this increase in number there appears to be a restriction of the vertical range of the different species, and a consequent increase of their value for determining different geological horizons. Rhynchonella nigricuns can only tell us that we are dealing with tertiary strata, and its presence indicates no particular division or group of strata—as eocene, miocene, or pliocene; and R. squamosa does not do more for our cretaceous strata; but in our oolitic, liassic, triassic, and permian rocks, the case is different, and each particular fossiliferous horizon may generally be determined by the presence of a peculiar species of Rhynchonella.
Hence the increased importance of the genus in these formations, and the necessity for studying those characters by which it may be identified. Some of the species from these older rocks appear as if considerably different from the ordinary types of the genus Rhychonella in outline and other external characters, and one or two species are readily mistaken for Terebratulidæ, more especially if the specimens are found as casts, thereby loosing the means for determining such forms by the impunctate structure of the shell.
Specimens thus liable to be mistaken are abundant in the triassic strata of the Kaihiku Range, Otago; and not until the discovery of testiferous specimens was it apparent that these did not belong to the genus Terebratula, which they closely resemble.
On making the discovery that the shell-structure of this species was impunctate, I at the same time observed that the hinge-teeth were minutely crenulated, or more properly denticulate, a character not seen in Terebratula. Further examinations showed that specimens, which without doubt belonged to Rhynchonella, had teeth marked in the same manner.
This discovery led to an examination of other and distinct species of Rhynchonella, in which this character was also seen.
When sufficiently well preserved to show it, this character proved present in every form of Rhynchonellæ which has been collected from the Secondary and Palæozoic rocks of New Zealand.
The recent forms R. nigricans, and R. psittacea, also show this character, and it has been detected in an English specimen from the lower greensand, at the base of the cretaceous system.
From the constancy of this character, I think there is sufficient grounds for regarding it as of generic value; as it has been found in all the species which afford the means of detecting it, and is seen in at least two of the living types of the genus.
I am not aware that hitherto attention has been called to the presence of this character as distinguishing the genus Rhynchenella.
From “Woodward's Manual of the Mollusca,” p. 376, I take the following description of the genus:
“Shell trigonal, acutely beaked, usually plaited; dorsal valve elevated in front, depressed at the sides, ventral valve flattened, or hollowed along the centre, hinge-plate supporting two slender curved lamellæ, dental-plates diverging.” and I add teeth crenulated or minutely denticulated.