MARINE MOLLUSCA OF THE UNITED STATES. 137 



Shell elongated, cylindrical, gaping at each end ; epidermis dark, horny, 

 extending beyond the margins ; umbones posterior ; hinge edentulous ; 

 ligament concealed ; pallial line obscure. Solemya. 



Genus SAXICAVA, Fleuriau de Bellevue. 

 Bull. Soc. Philom., No. 62. 1802. 



1. S. arctica, Linn. Figs. 310, 311, 312, 313. 



{My a.) Syst. Nat., edit. xii. 1113. 1767. 

 Mytilus rttgosus, Pennant, Brit. Zool., iv. 110. t, 63, f. 72. 1777. 

 Mytilus pholadis, Mull. Zool. Dan. t. 87, f. 1, 2, 3. 

 Saxicava distorta, Say, Journ. A. N. S., ii. 318. 1822. 



Shell irregularly oblong oval, the right valve projecting over the 

 left except at the shorter end, generally gaping ; beaks prominent, 

 from which diverge two ridges or elevated lines, one running near 

 the posterior dorsal margin, the other to the lower angle ; these 

 lines are more or less distinct, or obsolete. Surface coarsely 

 wrinkled ; epidermis thin, dingy yellow. 



Length 1 inch, height 15 mill. 



Whole Coast. {Occurs throughout the world.) 



This protean species cannot be describe!:! with any accuracy, 

 being modified by the substances into which it bores. It is of 

 universal distribution, and has, under its different aspects, received 

 no less than five generic and fifteen specific names. 



In the last edition ofGould's " Invertebrata of Massachusetts" 

 the S. arctica and S. ritgosa are separated as distinct species, but 

 the differences pointed out are not permanent, and are therefore 

 unreliable. 



Genus PANOP-ffiA, Menard. 

 Ann. du Mus., ix. 131. 1807. 



This genus, of about a dozen species, inhabits from low water 

 to 90 fathoms, and from northern seas to Mediterranean Sea and 

 Australia. 



1. P. Norvegica, Spengler. Figs. 314, 315. 



{Mya.) Skrivt., Nat. Selsk., iii. 46, t. 2, f. 18. 

 Glyeimerus arctica, Lamarck. Anim. s. Vert., edit. 2, vi. 70. 



Shell oblong, trapezoidal, thick, covered with a dark, rough epi- 

 dermis ; beaks anterior, the anterior and posterior margins oblique 

 and subparallel ; surface raised into tvo rounded, broad elevations 

 which proceed from the beaks to the basal angles, dividing the sur- 

 face into three nearly equal portions. 



Length 62.5, height 40, diam. 30 mill. 



New England, northwards. {Eur.) 



