VIII 



PHYLUM MOLLUSCOIDA 



361 



ventral valve, the hinge-teeth in their turn being received into de- 

 pressions (s) placed on each side of the cardinal process. The inner 

 surface of the dorsal valve is produced into a median ridge or 

 septum (sp\ continuous posteriorly with the cardinal process, and 

 attached on either side of the base of the latter are the two ends 

 of a delicate calcareous ribbon, the shelly loop (s. I), which projects 



G 



t/.v 



d.-m' 



cut, m. 



FIG. 289. IVIagellania flavescens. A, the entire sliell from the dorsal aspect, and B, from 

 the left side ; C, interior of ventral valve, and D, of dorsal valve, ad. m. adductor impres- 

 sions ; b. beak ; c. p. cardinal process ; d. deltidium ; d. m. divaricator impressions ; d. v. 

 dorsal valve ; /. foramen ; p. m. protractor impressions ; s. tooth-socket; s. I. shelly loop ; 

 sp. septum; t. hinge-tooth; v. aj. m. adjuster impressions; v. v. ventral valve. (After 

 Davidson.) 



freely into the cavity enclosed between the two valves, and has the 

 form of a simple loop bent upon itself. The inside of the dorsal 

 valve also has muscular impressions. 



Externally both valves present a series of concentric markings 

 parallel with the edge or gape : these are lines of growth, the 

 shell being built up by new layers being deposited within those 

 previously formed, and projecting beyond them so as to form a 

 series of outcrops. 



