GENUS RHYNCHONELLA — STENOCISMA. 335 



Genus or Subgenus Stenocisma (Conrad— Hall). 



Shells subtriangular, ovoid or subglobose; hinge-line short : beak of 

 ventral valve extended, attenuate or more or less arcuate and appres- 

 sed against the o'pposite valve. A mesial sinus and fold on the ventral 

 and dorsal valves respectively. Surface plicated with simple or rarely 

 bifurcating plications. 



The valves are articulated by teeth and sockets. A strong median sep- 

 tum in the dorsal valve of many of the species, which becomes thick- 

 ened in its upper part, with a shallow triangular pit in the centre, and 

 the crura supported on each side : dental sockets crenulate. The teeth are 

 strong, with lamellae extending to the bottom of the valve, limiting the 

 rostral cavity, and sometimes partially surrounding the muscular area. 

 In casts of the ventral valve, the cavities made by the diverging 

 lamellae are very distinct; and in the dorsal valve, the place of the sep- 

 tum is strongly marked, the space being wide above, and within this 

 cavity at the upper part there is a small angular elevation of greater or 

 less extent, which originally filled a depression in the upper part of the 

 septum. 



I would hy no means include all the palaeozoic so-called Ehynchonellfe under this 

 genus, for I uni satisfied that tiiere are plicated shells of similar form which have 

 a different intern.al structure ; but it is absolutely necessary that this structure be 

 known, before we can make a proper disposition of the species. 



RU} nclionella ( Stenocisnia ) tetliys. 



PLATE LIV. 

 / Rhynchonella tethyi : BiLLiNQS, Canadian Journal, p. 270. 1860. 



Shell subtrigonal ovate, usually wider than long, sometimes length and 



width equal. 

 Ventral valve depressed convex in the upper part, nearly flat at the 



sides : mesial sinus beginning above the middle, moderately depressed 



in young shells, and in older ones becoming deep in front and abruptly 



curving towards the dorsal valve. 



