NUCULANIDAE AND ArCIDAE 33 



Genus NFCULAJTA Link, 1807 

 >'uculana acuta^'^ (Conrad) PI. 1, fig. 5 



Alt., 5; length, 10 mm. Equivalve; extremely inequilateral; 

 posterior to the umbos, the shell is rostate and prolonged to an acute 

 tip; umbos small, closely approximate, and directed backward. Thin, 

 brownish periostracum; sculpture of well-mipressed grooves almost 

 parallel with the ventral margin but not extended over the slightly 

 depressed rostrum which bears fine longitudmal striae. Ventral 

 margin rounded and smooth, dorsal margm posterior to umbos con- 

 cavely curved; hinge angular, a triangle fossette below umbos; 

 numerous chevron-shaped teeth anterior and posterior to fossette; 

 anterior and posterior muscle scars; pallial line sinuated; interior 

 polished, not nacreous. 



There is an aberrant form of A'^. acuta differing from the typi- 

 cal in an entire absence of concentric sculpture. Specimens of each 

 form are commonly taken together. 



Dredged from sandy-mud bottom in one to six fathoms. 



Family AKCIDAE 



The family Arcidae is well represented from Maine to Florida 

 and the Gulf Coast by species living in relatively shallow water, 

 and by less common species taken at varying depths up to about 

 300 fathoms of water. Some members of this group creep about and 

 burrow in sand or mud; some frequent reefs and hard bottom; 

 many attach themselves to some stable point of support by a byssus 

 which can be cast off and quickly removed. Often many individuals 

 are attached to one Atrina and to each other. Shells of Atrina rigida 

 seem to be especially favored by A. zebra and A. umbonata. Young 

 Areas climb the sides of an aquarium by means of their byssal 

 threads, as do young Mytilus. 



The mollusks of this family are sensitive to changes in intensity 

 of light and will close the shell immediately when a shadow falls upon 



^•^ Lat., nucula, with ana, suffix meaning concerned with; acutus, from 

 acuere, to sharpen. 



