TEREBRATULIDiE. 573 



merly swarmed at the bottom of the sea, and played a most 

 important part among the molluscous population of those 

 primeval deeps. 



Fam. TEREBRATULID.E. 



Animals always attached by a peduncle, furnished with 

 cirrhated arms, united throughout by membrane, folded 

 upon themselves, and only spiral at their extremities. 



Shell minutely punctate, inequivalve, usually oval or sub- 

 circular, smooth or striated ; ventral valve the larger, the 

 beak produced and perforated at the apex; foramen sepa- 

 rated from the hinge-line by a triangular plate or delti- 

 dium usually composed of two pieces; teeth supported by 

 plates, one on each side of the deltidium ; dorsal valve 

 with a prominent cardinal process between the sockets for 

 the teeth, a hinge plate with four cavities, and a central 

 ridge or septum. Internal skeleton in the form of a slender 

 shelly loop attached by its crura to the hinge-plate and 

 furnished near its origin with oral processes. 



The strongly cirrhated, looped or contorted oral arms 

 fixed to a well-developed apophysary skeleton is the distin- 

 guishing characteristic of this family. The muscles which 

 close the shell arise from the centre of the ventral valve, 

 and are inserted at four points near the centre of the dorsal 

 valve ; besides these, there are retractor and peduncle mus- 

 cles. The Terebratulidce are all marine, and range from 

 low-water to 1 00 fathoms ; they are found in all seas. 



Sub-fam. TEREBRATULIN^E. 



Shell usually oval; valves convex, the margins entire or 



only slightly waved ; hinge-line curved; beak of the larger 

 vol. n. 4 E 



