888 INTRODUCTION TO CONCHOLOGY. 



ration or fissui'e in the lower valve. The valves are not 

 united by any ligament, nor do they exhibit, except in the 

 genus Thecidkim, the cardinal processes or internal frame- 

 work so characteristic of the TerehraUda. Their shell is 

 rather thin, — more horny than shelly. Three genera be- 

 long to this family — Thecidium, Crania, and Orhicula. 



Thecidiiim. — This genus, intermediate between the two 

 families of Brachiopods, — resembling the TerebraiulcB, not 

 only in shape, but in the cardinal processes, — differs in being 

 imperforate, and without either pedicle or tendon. It is 

 one of considerable interest to the geologist, and at the 

 time of its first introduction by Defrance was known only 

 in the fossil state. It is needless to speak further con- 

 cerning the small, ovate, inequivalve, and nearly equilateral 

 shell which pertains to the present genus, except to state 

 that, as regards the interior of the shell, both valves are of 

 a very pale greenish colour, and finely granulated, the lower 

 valve being convex, the upper fiat and curiously indented, 

 as if to fit certain corresponding parts in the body of the 

 animal. These indentations, which are spread round in a 

 semicircular direction from the hinge, look as if they were 

 pricked out in wax, and are occasionally filled with the dried 

 remains of numerous fine cilia. 



