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 condyle. ‘Ten species. 
Fossils only. | 
5. Genus Plicatula. Pl. IX. 
Animal. Unknown. s 
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 American seas. Five species. 
Plicatula ramosa. Plicatula cristata. 
P. depressa. P. Australis. 
P. reniformis. 
6. Genus Spondylus. Pl. IX. 
Animal. Body moderately compressed, provided inferiorly 
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 gedaropus. 
S, coccineus. S. arachnoides, 
S. spathuliferus, S. multilamellatus. 
