MARINE MOLLUSCA OF THE UNITED STATES. 139 



leap is performed by contracting the foot at the same time that 

 water is expelled from the posterior opening by closing the valves. 

 There are four known species ; burrowing in mud, in about two 

 fathoms watei*. 



1. S. velum, Say. Fig. 319. 



Journ. Philad. Acad., ii. 317. 1822. 

 Very thin and fragile, transversely oblong-elliptical ; beaks not 

 elevated, umbones scarcely apparent ; basal and hinge margins 

 parallel, ends rounded. Epidermis glossy, with radiating lines, 

 the edges fringed. Reddish brown, with light yellow radiations. 

 Length 1, height .5 inch. 



New England. 



2. S. borealis, Totten. Fig. 318. 



Am. Journ. Science, xxvi. 366, f. 1. 

 Shell fragile, oblong, but larger and more solid than the preced- 

 ing. Radiations with a larger free space ; the edges of the epider- 

 mis not rounded by the slits but preserving a square form, and 

 curved outwards ; the cartilage support not arched or vaulted, but 

 forked, with the hinder part directed obliquely forwards. Dark, 

 blackish-brown. 



Length 2.5, height .8 inches. 



New England. 



Family MYID^E. 



Animal with the mantle almost entirely closed ; pedal aperture 

 and foot small ; siphons united, partly or wholly retractile ; bran- 

 chiae two on each side, elongated. 



The shells gape usually at each extremity, and the cartilage is 

 contained in a spoon-shaped cavity at the hinge. Living in the 

 sand or mud, lying on the side. 



Synopsis of Genera. 

 Shell oblong, inequivalve, gaping at the ends, left valve smallest, with a 

 large, flattened, spoon-shaped cartilage process ; pallial sinus large. 



Mya. 



Shell thick, inequivalve, gibbous, closed, produced posteriorly ; right valve 

 with a prominent tooth in front of the cartilage pit ; left valve smaller, 

 with a projecting cartilage process ; pallial sinus slight ; pedal scars dis- 

 tinct from the adductor impressions. Cokbula. 



Shell globular, attenuated and gaping behind ; right valve a little smaller 

 than the left valve ; umbones strengthened internally by a rib on the pos- 



