168 BIVALVES. 



valve is convex, and is produced at the apex 

 into a projecting beak which appears as if it had 

 been sliced off by a sharp instrument, presenting 

 a triangular flattened surface, having a groove 

 which receives the decaying part of the ligament. 

 The shell is often eared. 



OBSERVATIONS ON THE SHELL AND ITS 

 INHABITANT. 



iThe shells of this genus are remarkable not 

 only for the brilliancy of colouring, which most 

 of them display, and for the singularity of their 

 external form, but also for the clearness and 

 precision of the characters which distinguish 

 them from the shells of other genera. The 

 roughness and irregularity of their appearance, 

 their two strong incurved teeth, and the inter- 

 mediate sinus for the ligament, are their dis- 

 criminative generic marks. The genus contains 

 only three species, one of which has not the 

 flattened beak and external ligament, and its 

 shells are remarkable for being plaited longitu- 

 dinally like a fan half opened ; they have been 

 separated by modern conchologists from the 

 Spondylus under the significant name of Plica- 

 tula.* The Spondyli adhere to rocks, corals, 

 and other marine substances, often in groups 

 forming large masses. The animal has the 

 edge of its mantle fringed with short tentacular 

 filament ; it has a small radiated foot. From a 



* Plicatula, a little fold, from plica, a fold. Plate VIII. fig. 10 



