334 
PALAEONTOLOGY OF NEW-YORK-: 
The recent species of Rhynchonella do not certainly furnish any evidence of 
similar characters, so far as I have been able to examine specimens or illustrations. 
The dental plates of the ventral valve are not, it is true, essentially different in 
fossil and recent Rhynchonella ; but the same comparison may be made with 
other genera even out of the Family Rhynchonellid.®. 
With these facts before us, I propose to revive the name Stenocisma* of Con¬ 
rad for the species under consideration, extending the term to include the typical 
species Rhynchonella formosa of the Lower Heidelberg 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 Terebratula is wholly unknown, and the shells usually referred to that genus 
“ I propose to group under the generic name of Stenocisma, derived from two Greek words signi- 
“ fying narrow fissure, a character these shells possess under the imperforate apex of the larger 
“ valve, and which serves to connect the genus with Delthyris, from which it differs in having no 
“ cardinal area. This last-named character, on the other hand, connects it with the Genus Strygoce- 
“ phalus. I refer to it the common Silurian bivalve, Terebratula schlotheimii, Yon Buch.” 
Notwithstanding the assertion 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 
growth a perforate apex. It was also a Lower Helderberg species ( Rhynchonella formosa) which was 
referred by Mr. Conrad to “ the common Silurian bivalve T. schlotheimii.” I have in my possession 
a lithographed plate of the fossils of the Lower Helderberg group by Sir. Conrad, 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 Terebratula schlotheimii. 
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 Hemithyris, 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. angustifrons, M'Coy, H. subundata, M‘Coy ; H. hemispherica, var. scotica, M'Coy ; all figured on 
the same plate, and belonging to three distinct genera; the first named being undoubtedly a Meris- 
tella, and having internal spires. This generic term, therefore, cannot he adopted unless redefined 
and very much restricted in its application. 
