GENERA OF SHELLS. 55 



superior valve convex, sub-gibbous, with two 

 prominent callosities within. 



The three holes in the crania do not seem to perfo- 

 rate it completely, unless by accident, when removed 

 from the body to which it was fixed by the outer 

 surface ; hence they cannot be the issues of muscles. 

 These holes give the lower valve the appearance of a 

 death's head. 



C. personata. 



BRACHIOPODA. 



The shell of the brachiopoda is more or less inequi- 

 valve, and opens by a hinge ; the true ligament of the 

 valve is not known. The shell always adheres to 

 marine bodies. 



Orbicula. Shell suborbicular, inequivalve ; 

 no conspicuous hinge ; inferior valve very thin, 

 flat, adhering to marine bodies ; superior sub- 

 conic with an acute vertex more or less promi- 

 nent. 



The lower valve is sometimes so thin as to be 

 scarcely perceptible, whence Muller supposed it to be 

 a univ^alve shell, and referred it to the patellae. 



O. norvegica. 



Terebratula. Shell inequivalve, regular, 

 subtriangular, attached to marine bodies by a 

 short, tendinous pedicle ; beak of the larger 

 valve produced, often curved, perforated at 

 summit by a round hole or a notch ; hinge 

 with two teeth ; two nearly osseous, slender, 

 elevated, forked, variously ramified branches 



