Crania, Retzius. 



Gen. Char. An unequal valved bivalve, slightly 

 irregular, suborbicular ; upper valve patel- 

 liform ; lower valve attached, flattish ; mus- 

 cular impressions four; no hinge; animal 

 without a byssus. 



An this Genus the thickness of the lower or attached 

 ■valve is very variable, even in the same species it is 

 sometimes so thin as to have been overlooked ; never- 

 theless it is that valve which possesses the characters of 

 the Genus in the most eminent degree ; for, it is found 

 to be attached, like the lower valve of Ostrea, by its own 

 surface ; to be Avholly destitute of hinge, by which the 

 other valve is united to it, either in the form of teeth that 

 lock into each other, or of ligament upon the edge ; to be 

 marked with the impressions of four tendinous muscles, 

 that unite it at once with the animal and the other valve ; 

 and to have a granulated inner surface, especially about 

 the margin, which arises from its cellular, and not foli- 

 ated structure, a structure possessed also by Radiolites 

 and Calceola, which Lamarck has arranged with it un- 

 der his family " Rudistes." Of the four muscular im- 

 pressions, two are placed at a distance f/om each other, 

 near the margin, which is generally almost straight be- 

 tween them ; the others are towards the centre of the 

 disk, and only separated from each other by an eminence 

 in the lower valve, while they are more distant in the 

 upper ; the free or convex valve has a nearly central 

 apex, is generally thin, and has the same granulated 

 surface near the edge as the fixed valve has, but not in so 

 remarkable a degree. 

 Vol. V. June 1823. i . r/ 



