PELECYPODA 



597 



C. siliqua (Sprengler). Shell very thick and heavy; length 85 mm.; 

 height 35 mm.; width 24 mm.; periostracurn thick and black: Cape Cod 

 to the Arctic Ocean, in deep water. 



SUBORDER 6. PHOLADACEA. 



Shell more or less modified by the boring habit; ligament and hinge 

 absent; mantle closed; siphons very long and united almost their whole 

 length ; foot short, often more or less rudimentary ; 3 adductor muscles : 2 

 families, marine, the animals boring long passages in wood, rock, or clay 

 by means of a rasping motion of the shell. 



Key to the families of Pholadacea: 



o t Shell large ; siphons long 1. PHOLADIDAE 



a. Shell small; siphons many times as long as shell; shipworms. .2. TEREDIDAE 



FAMILY 1. PHOLADIDAE. 



Shell rather large, thin, fragile, white, gaping at both ends, armed 

 in front with rasp-like ridges; pallial sinus deep; dorsal margin of shell 

 reflected in part over the umbo ; one or more accessory dorsal valves ; an 

 interior curved muscle process extending from the umbonal cavity: 80 

 species, which bore symmetrical holes in clay, wood, or rocks; about 12 

 species on the Atlantic coast. 



Key to the genera of Pholadidae here described: 



a t Dorsal margin of shell reflected over the umbo 1. PHOLAS 



a, Dorsal margin but slightly reflected 2. ZIRPH^EA 



1. PHOLAS L. Shell elongated and very convex; dorsal margin pro- 

 tected by 2 accessory valves ; foot large : 20 species, in all seas. 



Fig. 942 Fig. 943 



Fig. 942 -Pholas costata (Gould). Fig. 943 Pholas truncata (Gould). 



P. costata L. (Fig. 942). Shell large, 15 cm. long, 5 cm. high, and 

 5 cm. wide, with about 30 radiating, serrated ribs, the interior being 

 marked by corresponding depressions; umbo near anterior end; acces- 

 sory valves horny, not calcareous : Cape Cod to South America, forming 

 deep burrows in the clay; common in the southern states, rare towards 

 the north; Europe. 



