80 CONCHOLOGY. 



Shell. Moderately thick, regular, free, subequivalve, sub- 

 auriculated, the two valves nearly equally dilated, both provided 

 with a distinct summit reflexed to the middle of a plane surface, 

 with a large triangular slope in the middle ; articulation trans- 

 verse, straight, and by two distant lateral condylse. Ten species. 

 Fossils only. 



5. Genus Plicatula. PI. IX. 

 Animal. Unknown. 



Shell. Solid, adhering, subirregular, 'inauriculated, inequi- 

 valve, pointed at the summit, rounded and subplicated posteriorly ; 

 inferior valve without heel; hinge cephalic, longitudinal, pro- 

 vided upon each valve with two strong teeth, entering in corres- 

 ponding cavities ; ligament altogether internal and inserted in a 

 median cavity. Inhabits the i^merican seas. Five species. 

 Plicatula ramosa. Plicatula cristata. 



P. depressa. P. Australis. 



P. reniformis. 



6. Genus Spondylus. PL IX. 



Animal. Body moderately compressed, provided interiorly 

 with a rudiment of a foot, without byssus ; mantle open in all its 

 inferior and superior portion ; mouth surrounded with very thick 

 and fringed lips. 



Shell. Solid, adhering, subregular, more or less spined, sub- 

 auriculated, inequivalve; the right or inferior valve fixed, much 

 more excavated than the other, and having posteriorly at the 

 summit a triangular face enlarging, and elongating with age ; 

 hinge longitudinal, provided in each valve with two strong teeth 

 entering corresponding cavities ; ligament short, nearly median, 

 partly exterior ; muscular impression single and subdorsal. Found 

 in all the seas of hot climates, and even in the Mediterranean. 

 Four or five fossils are found in France, one in South America. 

 Twenty-one species. 



Spondylus candidus. Spondylus gsedaropus. 



S. coccineus. S. arachnoides. 



S. spathuliferus. S. multilamcllatus. 



