i;J2 PALEONTOLOGY OF NEW-YORK. 



of these forms becomes entirely spinous, they have assumed other characters essentially 

 different from Lept.€;na, 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. 



TcrebratuHtes bifqiatvs. Schlotheim, 1820, Petrefacta, p. 265. 

 Atrypa dorsata? Hisinger. 1S37, Lethaea Suecica, pag. 76, pi. 21, fig. 14. 

 Spiri/er biforatus. F.ichwald, 1S40, .Sil. System in Esthland, p. 144. 

 — — MuRcHisoN & Vehneuil, 184.'), Geol. and Pal. Russia, &c. Vol. li. p. 1.35. 



Vai'. A. Spirifcr lynx, Eichvv.ild. 

 Var. B. — chiitahiSy Pander. 

 Var. C. — chamu, 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 Buck, the existence of five plaits 

 in tlie sinus, and the greater propoitional width of the shell. These characters alone are 

 scarcely sufficient to constitute a variety ; for we have specimens willi five plaits in tlie 

 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 ditTerent 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 throe varieties cited ; but there are so many intermediate forms, tliat 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. 



