S34 PALEONTOLOGY OF NEW-YORK. 



The recent species of Rhynchonella do not certainly furnish any evidence of 

 siuiilur characters, so far as I have been able to examine specimens or ilhistrations. 

 The dental phites of the ventral valve are not, it is true, essentially dift'eront in 

 fossil and recent RinNCiiONEiXA ; but the same comparison may bo madi; with 

 other genera even out of the Family Hiiynchonellid^. 



With these facts before us, I propose to revive the name Stenocisma* of Con- 

 KAD for the species under consideration, extending the term to include the typical 

 species Rhi/nchonella formosa of the Lower Helderbcrg group. This genus may 

 be characterized as follows : 



•In his second Annual Report (page 59, 1839), Mr. Conrad, speaking of the rocks of New- York, 

 says : "The Genus Terkbbatula is wholly unknown, and the shells usually referred to that genus 

 " I propose to group under the generic name of Stknocisma, derived from two Greek words signi- 

 " fying narrow fissure, a character these shells possess under the imperforate apex of the larger 

 " ralve, and which serves to connect the genus with Deltdyris, from which it diflers in having no 

 " cardinal area. This last-named character, on the other hand, connects it with the Genus Strygock- 

 " PHALUS. I refer to it the common Silurian bivalve, Ttrtbratula schlotheimii, Von Buck." 



Notwithstanding the a.ssertion of imperforate apex, we have learned that many of these forms, and 

 probably all those which have been referred to Terebratula, have had at some period of their 

 g;rowth a perforate aj)cx. It was also a Lower Helderlxirg species (^Rhynchonella formosa) which was 

 referred by. Mr. Conrad to "the common Silurian bivalve T. sMolhetmii." I have in my po.sscssion 

 a lithographed plate of the fossils of the Lower Helderberg group by Mr. Conbad, with, the names, in 

 his own hand, written beneath the figures; the species I have since designated as R. formosa having 

 there been identified with Terebratida schlotheimn. 



Although the generic characters were not fully described, and with imperfect reference to species, I 

 think it preferable to adopt this name instead of introducing a new one. 



The name IIemithtbis, applied by some authors to certain rhynchonelloid forms, has been used to 

 include very heterogeneous materials ; and without citing a long list to prove this, I may mention 

 H.angutlifrons, M'CoY, H. subundtUa, M'Cor; H. hemisphericff., var. seotica, M'Coy' ; all figured on 

 the same plate, and belonging to three distinct genera; the first named being undoubtedly a Meris- 

 TKLi.A, and having internal spires. This generic term, therefore, cannot be adopted unless redefined 

 •nd very much restricted in its application. • 



