454 



MOLLUSCA. 



usually much more convex than the other, and its general 

 form being compact and not extended; while the hinge-line is 

 often shorter than the greatest width of the shell. The mus- 

 cular scars are quadrate, and not extended either vertically or 

 laterally. Lastly, the " cardinal process "- that is, the pro- 

 jection of the dorsal valve to which the " cardinal muscles " 

 are attached is undivided and linear. Though the species 

 of Orthis most nearly resemble certain forms of Strophomena, 

 and are very liable to be confounded with these, one or two 

 species (such as Orthis biforata, fig. 302, a) closely simulate 

 the genus Spirifera in general form. 



In the genus Orthisina (fig. 304) the shell nearly re- 

 sembles that of Ortliis ; but there is a double hinge -area, 

 largest in the ventral valve, the central 

 fissures of which are always covered by a 

 convex deltidium ; whereas in the latter 

 genus they are open. In some species (as 

 in 0. Verneuili) the deltidium is perforated 

 by a foramen under the beak of the ventral 

 valve. The typical species of Orthisina 

 are Silurian ; but the genus is stated by Mr 

 Davidson to range through the Devonian 

 and Carboniferous into the Permian. 



In Strophomena (figs. 302, 303, and 305) the shell is de- 

 pressed, generally semicircular, the hinge-line as long as the 

 'width of the shell, or longer. The surface may be smooth, 

 but is most commonly striated or ribbed. There is a double 



Fig. 304. Orthisina 

 Verneuili. Lower Silu- 

 rian. 



Fig. 305. Strophomena antiqimta. Silurian. 



hinge-area, which is largest in the ventral valve. Each hinge- 

 area has a median notch, which, in the ventral valve, is par- 

 tially covered by a deltidium. The ventral valve may be 



