228 



MOLLUSCA. 



tinguished by having an external ligament. The posterior side 

 of the shell is generally the shortest, but the reverse is some- 

 times the case. 



The shells of the genus Cyrtodonta (Palaarca of Hall) are 

 very inequilateral, the umbones being anterior (fig. 186). The 



Fig. 186. Cyrtodonta Hindi (Billings). Lower Silurian, a Dorsal view ; b Side view. 



hinge-area is undefined, and the surface generally smooth. 

 There are a few (three) anterior cardinal teeth, and " two or 

 three remote oblique posterior teeth parallel to the hinge- 

 margin " (Salter). The species of Cyrtodonta appear to be 

 exclusively confined to the Silurian and Devonian Rocks. 



In the genus Leda the shell resembles that of Nucitla, especi- 

 ally in having numerous teeth on either side of a central 

 cartilage-pit. The shell, however, is oblong, rounded in front, 

 and pointed behind. The occurrence of Leda in the Palaeozoic 

 period is dubious ; but numerous species are known from the 

 Secondary and Tertiary Rocks. 



FAM. 5. TRIGONIAD^. Shell equivalve, trigonal ; hinge-teeth 

 few, diverging ; umbones directed posteriorly. Mantle open ; 

 foot long and bent. The most important genera of this family 

 are Trigonia, Myophoria, and Axinus. 



