188 PALAEONTOLOGY OF NEW YORK. 



and we should feel justified in assuming Rhynchonella formosa to be the type of 

 the genus rather than to reject the well-established term Camarophokia and 

 substitute Stenoschisma for it. 



Upon reviewing the Devonian shells which were referred to Stenoschisma 

 in Volume IV of the Palaeontology of New York, it has become evident that 

 some modification may be necessary in the application of that term. The in- 

 ternal structure of the type-species R. formosa was not at that time demon- 

 strated, nor, so far as we are aware, has it since been determined with the 

 accuracy now required. The Stenoschismas of Volume IV are characterized 

 by a strong septum in the brachial valve, cleft posteriorly, each lateral branch 

 supporting one of the crural bases ; the latter are thus separated medially by 

 the triangular cleft whose base is the divided median septum. An elongate 

 umbonal cavity is thus formed beneath the hinge-plate, and this must be re- 

 garded of some morphological significance in the relation of these shells to 

 Camarophobia and its allies. The species possess no cardinal process, and the 

 teeth are supported by parallel vertical dental lamellae. 



These characters are not shared by Rhynchonella formosa. In this species all 

 our preparations show that the septum of the brachial valve is represented only 

 by an exceedingly obscure median thickening, being in fact virtually wanting; 

 there is no such median subcardinal cavity as above described, but the hinge- 

 plate is divided by a fissure which extends to the bottom of the shell, and 

 contains a slender longitudinal cardinal process; the divisions of the hinge- 

 plate are not large, have concave upper surfaces, and the crura, which are long, 

 recurved and expanded at their extremities, are produced from the inner angles 

 of these divisions without interruption. The dental lamellae of the pedicle- 

 valve are short and convergent. The surfaces of the dental sockets, which in 

 the Devonian species are sometimes crenulated, are here smooth. It is neces- 

 sary to recognize the importance of these palpable difierences in R. formosa and 

 the group of shells ascribed to Stenoschisma in the work referred to, and it 

 seems necessary to render to this genus a stricter construction than it has here- 

 tofore received. In so doing R. formosa will stand as the only known repre- 

 sentative of the type of structure described, and we are inclined to recognize 



