BRACHIOPODA. 2O? 



the single species K. Leonhardi of the Trias of St Cassian. 

 The shell resembles Producta in being eared, and the dorsal 

 valve is concave, and follows the curve of the ventral valve. 

 It differs from Producta in having the arms supported upon 

 spiral processes. 



FAM. V. RHYNCHONELLID^E. Animal free, or attached by a 

 muscular peduncle issuing from an aperture situated under the 

 extremity of the beak of the ventral valve. Arms spirally 

 rolled, flexible, and supported only at their origin by a pair of 

 short, curved, shelly processes. Shell-structure fibrous and 

 impunctate. (Fig. 151.) 



Fig. 151. R hyuchonella capax ; dorsal, profile, and ventral views. 

 Lower Silurian. 



The Rhynchonellida 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 Palaeozoic. 

 The genus Rhynchonella (fig. 151) 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. 139), 

 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 Rhynchonella are very 

 abundant in both Palaeozoic and Mesozoic deposits, and two 

 species are known at the present day. 



In the genus Pentamerus (fig. 152), the shell is ovate, the 

 valves articulated by teeth and sockets, generally ribbed or 

 striated, but sometimes smooth. 



The beaks are incurved, that of the ventral valve concealing 

 a triangular fissure. Inside the ventral valve " two contiguous 

 vertical septa coalesce into one median plate, extending from 

 the beak to a greater or less distance ; and then diverge and 

 form the dental plates, enclosing a triangular chamber of much 

 smaller dimensions than the lateral ones." (Davidson.) The 

 small central chamber must have been occupied by the diges- 

 tive organs, and the spiral arms must have filled the great 



