{3:2 PALEONTOLOGY OF NEW-YORK. 
of these forms becomes entirely spinous, they have assumed other characters essentially 
different from Leprana, and become the true Propuctus. In the same manner the ArryPA 
of the older rocks, having no visible foramen or perforated beak, passes through some other 
modifications, and gradually assumes the characters of the true TerrpratuLa. At the 
same time, the deviations from the type of Arrypa give rise to the Genera PENTAMERUs, 
StricocePHaLus, &c.; while some of these approach the DeLtuyris, which never departs 
from its normal character to such a degree as to warrant a generic distinction, according to 
our present knowledge of these forms. 
DELTHYRIS BIFORATUS. 
Tercbratulites biforatus. ScuLorurm, 1820, Petrefacta, p. 265. 
Atrypa dorsata? Histncer, 1837, Lethea Suecica, pag. 76, pl. 21, fig. 14. 
Spirifer biforatus. FExcuwaxn, 1840, Sil. System in Esthland, p. 144. 
— — Murcuison & Vrerneui, 1845, Geol. and Pal. Russia, &c. Vol. 1. p. 135 
Var. A. Spirifer lynx, Ercuw any. 
Var. B. — dentatus, PANDER. 
Var. C, — chama, ErcHwaAup. 
The synonymy above is cited from the work of Messrs. Murcuison and pe VERNEUIL, 
on the Geology and Paleontology of Russia and the Ural Mountains. Under this species, 
three principal varieties are designated, while the 7. biforatus of ScHLOTHEIM 1s very rare 
both in Europe and elsewhere. The characters by which that one is distinguished from the 
other principal varieties, appear to be, according to von Buen, the existence of five plaits 
in the sinus, and the greater propoitional width of the shell. These characters alone are 
scarcely sufficient to constitute a variety; for we have specimens with five plaits in the 
sinus, where the width is not extended ; while we have others with this number of plaits, 
where the width is extended ; and again we have specimens with a fewer number of plaits 
in the sinus, where the width is much extended : so that it is scarcely possible to find any 
constant variety in this respect. 
The examination of more than one hundred specimens from different localities has 
scarcely shown two precisely alike in every particular, except the very young shells, which, 
both in New-York and in western localities, often present the same essential characters in 
every respect. We are able to distinguish, in our American specimens, at least an approach 
to the three varieties cited ; but there are so many intermediate forms, that strict distinctions 
cannot be drawn. This we should expect, when we notice the fact just stated of the extreme 
variation in the external characters, so that we can scarcely hope ever to find constant 
forms in any one of the varieties : neither should this be expected, for a constant variety 
would compel us to admit a specific distinction. 
