132 
PALEONTOLOGY OF NEW-YOKK. 
of these forms becomes entirely spinous, they have assumed other characters essentially 
different from Leptena, and become the true Productus. In the same manner the Atrypa 
of the older rocks, having no visible foramen or perforated beak, passes through some other 
modifications, and gradually assumes the characters of the true Terebratula. At the 
same time, the deviations from the type of Atrypa give rise to the Genera Pentamerus, 
Strigocephalus, &c.; while some of these approach the Delthyris, 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. 
Tcrebratuiites biforatus. Schlotheim, 1820, Petrefacta, p. 265. 
Atrypa dorsata? Hisinger, 1837, Lethaea Suecica, pag. 76, pi. 21, fig. 14. 
Spirifer biforatus. Eichwald, 1840, Sil. System in Esthland, p. 144. 
— — Murchison & Verneuil, 1845, Geol. and Pal. Russia, &e. Vol. ii. p. 135. 
Var. A. Spirifer lynx , Eichwald. 
Var. B. — dentatus, Pander. 
Var. C. — chama, Eichwald. 
The synonymy above is cited from the work of Messrs. Murchison and de Verneuil^ 
on the Geology and Palaeontology of Russia and the Ural Mountains. Under this species, 
three principal varieties are designated, while the T. biforatus of Schlotheim is 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 Buch, 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, whichj 
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. 
