CONCHOLOGY. • 83 



4. Plagiostoma. Ten fossil species. 



Only known as fossils ; probably introduced here by La- 

 marck to serve as a connecting link for the genera Lima, 

 Pecten, Spondylus, and Podopsis. 



Shell rather thick, regular, free, sub-equivalve, sub-auricu- 

 lated ; the two valves almost equally convex, both provided 

 with a distinct summit, recurved in the middle of a level sur- 

 face, with a great triangular slope in the middle ; the cardi- 

 nal base transverse, straight ; hinge without teeth ; a conical 

 cardinal pit situated below the beak, partly internal, opening 

 outward, and receiving the ligament. 



P. spinosa. The thorny Plagiostoma. 

 Subarcuated, the umbo of one shell higher than that of 

 the other, with longitudinal ribs and remote concentric rings. 



5. Plicatula. Five species. 

 Separated from the Spondylus of Linnaeus on account of 

 its distinct structure. The ligament is altogether internal ; 

 it is without ears, and the prolonged beak so conspicuous 

 in that genus. The Plicatula has the faculty of affixing it- 

 self to another body, so that many are found grouped togeth- 

 er in clusters. The valves are strongly plaited within and 

 without, closely interlocking with each other. 



Shell solid, adhering, sub-irregular, without ears, inequi- 

 valve, pointed at the summit, rounded and plaited behind ; 

 hinge with two strong teeth in each valve, with a cavity be- 

 tween them, in which the ligament is internally inserted. 

 Plicatula ramosa. Plicatula cristata. 



P. depressa. P. reniformis. 



P. Australis. 



P. ramosa. The branched Plicatula. PI. 15, fig. 2. 



Oblong, trigonal, very thick ; strong longitudinal plaits ; 

 exterior brown, with a yellow tinge, with reddish arrow- 

 shaped markings ; interior white. 



