BRACHIOPODA. 449 



pair of short, curved, shelly processes. Shell-structure fibrous 

 and impunctate. 



Fig. ZQl.Rhynclionella capax ; dorsal, profile, and ventral views. Lower Silurian. 



The Rhynclionellidce range from the Lower Silurian to the 

 present day, in the person of the genus Rhynchonella itself; 

 but the remaining genera of the family are exclusively Palseo- 

 zoic. The genus Ehynchonella (fig. 294) has the valves more 

 or less convex, smooth or plaited, united by teeth and sockets. 

 The shell is trigonal, generally with a mesial fold and sinus 

 (fig. 275), and having the beak of the ventral valve acute, 

 incurved, or prominent. The foramen is situated under the 

 beak, open to view, or concealed, and entirely or partially 

 completed by a deltidium. The species of the genus Ehyn- 

 chonella are very abundant in both Palaeozoic and Mesozoic 

 deposits, and two species are known at the present day. 



In Leiorhynchus (fig. 295) are various Devonian Brachio- 

 pods, very closely allied to Rhynchonella proper, but having 



Fig. tab. Leiorhynchus Huronensis, viewed dorsally (a), ventrally (b), -aiul in profile (c). 

 Devonian. (Original.) 



the plications of the shell obsolete on the lateral angles, 

 while well marked on the mesial fold and sinus. The beak 

 of the ventral valve is pierced by a foramen, in at any rate 

 the early stages of growth, and there is a well-defined septum 

 in the dorsal valve. Triplesia (Silurian) has a triangular 

 VOL. I. 2 F 



