Lamarck's Genera of Shells. 41 



Type. Plicatula cristata *. 

 Shell oblong, cuneiform, ferruginous, subcristate ; folds large, 

 simple, squamose. 



American Seas. 1 1 Species. PI. II. Fig. 90. 



6. Spondylusf. 



Shell inequivalve, adhering auriculated, spinous or rough; 

 beaks unequal ; an external, flattened, cardinal facet on the 

 lower valve, divided by a furrow. Two strong cardinal teeth 

 on each valve, with an intermediate pit for the ligament, com- 

 municating at its base with the external furrow. Ligament in- 

 ternal ; remains of former ligaments perceptible externally in 

 the furrow. 



The spondyli are particularly distinguished from the ostrese 

 by the cardinal teeth ; they are generally covered with spines, 

 which are occasionally very large, subulate, or lingular ; some- 

 times simple, sometimes foliated at their summit, and always dis- 

 posed in rows, or longitudinal, radiating striae, or ribs. They are 

 for the most part variously and brilliantly coloured ; the lower valve 

 is always the largest and most convex, and is terminated at the 

 beak, by a kind of talus, which appears as if cut with a sharp 

 instrument, and presents a flattened, inclined, triangular facet, 

 divided by a furrow. This cardinal area increases in length 

 by age, in consequence of the animal changing its place in the 

 shell as it grows, and at the same time displacing the upper 

 valve J. 



The animal, like that of the pecten, has two rows of short, 

 tentacular threads on the border of the mantle, and the vestige 



. ♦ Crested. Lamarck's third species. His type is P. ramosa. 



* ZttovJuXoc, spondylus, a knuckle, or vertebra. Also the original Latin name 

 for a kind of shell-Qsh. 



t On this supposed dislocation of the upper valve, Mr. Sowerby very 

 pertinently remarks, " The teeth of the two valves are so formed, that with- 

 out breaking away some portions of them, or of the circumjacent parts 

 of the hinge, the two valves cannot be separated. We have mentioned this 

 fact before in our account of the genus ostrea ; and we here repeat it, to 

 shew how impossible it is that the animal should displace its upper valve, 

 as Lamarck asserts, in order to produce the progressive elongation of the 

 area of the hinge of the lower valve." (Genera of Recent and Fossil 

 Shells. No. 9.) 



