326 



C(ELHELMINTHES. 



other dorsal, their free edges bearing bristles. Each mantle 

 secretes a shell largely composed of carbonate and phosphate of 

 lime. In a few the dorsal and ventral shells are similar, but 

 usually the ventral valve (in Crania attached directly without the 



FIG. 299. Waldheimia flavescens. (From Zittel.) Shell with arms and muscles, a, arm 

 with fringed border (/i); c, c', divaricators; d, adductors; D, hinge process (the 

 vertical line shows position of hinge). 



intervention of a stalk) is more strongly arched and has an opening 

 at the posterior end for the passage of the stalk (figs. 299, 300). 

 The flatter dorsal valve frequently bears a characteristic feature in 

 the skeleton of the arms (fig. 300) which, when present, has greatly 



FIG. 300. Waldheimia flavescens. (From Zittel.) A, dorsal, 5, ventral valve; a, b, c, 

 impressions of muscular insertions; a, adductors; h", adjusters (stalk muscles) ; 

 r, c', divaricators; s, hinge groove of upper valve in which the tooth (t) of the 

 lower valve passes ; Z, support of arms; d, deltidium; /, foramen for stalk. 



different expression. Its basis consists of two calcareous rods 

 which, bilaterally symmetrical, project downwards from the dorsal 

 valve. These may be connected by a curved transverse band, and 

 from their ends a spiral process may extend on either side. This 

 apparatus supports the spiral arms. When closed the valves com- 

 pletely enclose the body. When they open the gape is anterior, 



