50 Lamarck's Genera of Shells. 



These holes give the lo\ver valve the appearance of a death's 

 head*. 



Type. Crania perso7iataf. (Anomia craniolaris, Linn.) 



Shell orbicular ; the more gibbous valve conico-convex ; the 

 flatter, with three little pitsat the base.' 



Indian Seas. The only recent species known — the other four 

 species are fossil. PI. II. Fig. 102. 

 2d Family. 

 Brachiopoda X, (3 genera.) 



Conchifera with two opposite, elongated, fringed arms, near 

 the mouth, which are rolled up in a spiral form, and enclosed in 

 the shell > when in a state of rest. These are peculiar to the 

 brachiopoda. Mantle with two lobes, separated in front, en- 

 veloping or covering the body. 



Shell bivalve, adhering to marine bodies, either directly, or 

 by a tendinous cord. 



The shell of the brachiopoda is more or less inoquivalve, and 

 opens by a hinge. The true ligament of the valves is not known ; 

 the tendinous cord is merely a prolongation of the muscular 

 attachment of the animal, and does not assist in opening the 

 valves. The shell always adheres to marine bodies. This is 

 the last family of the conchifera. 



1. Orbicula§. 



Shell suborbicular, inequivalve ; no apparent hinge. Lower 

 valve very thin, nearly flat, adhering to marine substances ; 

 upper valve subconical ; summit more or less elevated. 



The lower valve of the orbicula is sometimes so thin as to be 



• Mr. Sowerby finds that these holes are muscular impressions, and that 

 they are four in number, instead of three, though two of them are so near 

 together, that he is not surprised that Lamarck, on a slight examination* 

 " should have described the genus Crania as having, in the lower valve, 

 three oblique perforations." He suggests the following as an amended 

 generic character of this shell. " Crania. — Bivalve, inequivalve, nearly 

 orbicular, compressed, fixed ; upper valve patelliform, with four internal 

 muscular impressions ; lower valve adhering, nearly flat, with four corre- 

 sponding muscular impressions, two near the centre, approximating and 

 nearly united, and two near the posterior margin, distant. No hinge, — 

 Trans. Linn., Soc. xiii. Mr. Sowerby discovered the two fringed arms, pe- 

 culiar to the brachiopeda, in a Crania from Shetland. It should therefore, 

 as he observes, evidently be transferred to that family. 



•t 3Iasked. t From $fax^tm, an arm, and woi/j, a foot. 



5 Orhiadus, a little round ball. 



