34 Marine Shells of the Western Coast ok Florida 



it. This light sensitiveness is a protection from birds flying above and 

 against the near approach of predatory enemies in the sea. Areas 

 are said to be the principal food of Murex, and the shallow water 

 species are liked by fish and sea gulls. 



The blood of moUusks, as a rule, is colorless, but some of this 

 group have red blood whose color is due to a compound of iron. 



Shells of the Arcidae may be equivalve or not equivalve, and 

 are usually thick, with a heavy periostracum. They are radiately 

 ribbed and sometimes cancellated. The umbos are directed forward 

 in most genera and more or less separate from each other by a flat, 

 rhomboidal area which is engraved with lines m various angular 

 patterns. The margins may be smooth or dentate, some times crenate 

 withm. The hinge is characteristic, a row of comblike teeth along 

 the dorsal margin of each valve for the greater part of its length. The 

 inner surfaces of the valves show unequal anterior and posterior 

 muscular impressions, a pallial line but no pallial smus. 



Genus ARCA Linne, 1758 



Subgenus ARCA s.s. 



Arcai* zebra Swainson {A. occidrntalis Philippi) PI. 2, figs. 9a, b 



Alt., 50; length, 90; diam., 43 mm. Yellowish-white, streaked 

 and patterned with brown, occasional albino specimens; heavy, 

 shaggy, brown epidermis; equivalve; inequilateral; umbos slightly 

 incurved, widely separated; hinge area flat and broad; hinge margin 

 straight; anterior and posterior margins oblique, posterior margin 

 sinuous; ventral margin slightly rounded with wide byssal notch 

 below umbos. About 26 distinct, narrow, rounded ribs with finely 

 ribbed, flat interspaces; fine lamellar ridges cross both ribs and 

 interspaces; hinge typical; interior smooth and purple colored; 

 margins defined by a purplish-brown, polished band, slightly in- 

 dented posteriorly; muscular and pallial impressions clear; a strong 

 thick byssus. 



From three to seven fathoms. 

 ^* Lat., area, a tliest. 



