354 



ZOOLOGY 



SECT. 



The dorsal valve (D) has no beak, but its posterior edge forms 

 a hinge-line which is produced in the middle into a strong cardinal 

 process (c. p) with a curiously folded surface : when the two valves 

 are in position this process fits between the hinge-teeth of the 

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

 depressions (s) placed on.qacji side of the cardinal process. The 

 inner surface of thdon&f valve is produced into a median ridge or 



af.tn' 



FIG. 295. Magellania flavescens. A, the entire shell from the dorsal aspect, and B, from 

 the left side ; C, interior of ventral valve, and D, of dorsal valve, ad. in. 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.) 



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 

 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 



