MARINE MOLLUSCA OF THE UNITED STATES. 13% 
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. 
(Mya.) Syst. Nat., edit. xii. 1118. 1767. 
Mytilus rugosus, Pennant, Brit. Zool., iv. 110. t. 68, f. 72. 1777. 
Mytilus pholadis, Mill. Zool. Dan. t. 87, f. 1, 2, 8. 
Sazicava distorta, Say, Journ. A. N. §., 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 described 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 of Gould’s “ Invertebrata of Massachusetts” 
the S. arctica and S. rugosa are separated as distinct species, but 
the differences pointed out are not permanent, and are therefore 
unreliable. 
Genus PANOPZA, 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. Norveerca, Spengler. Figs. 314, 315. 
(Mya.) Skrivt., Nat. Selsk., iii. 46, t. 2, f. 18. 
Glycimerus 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 two 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. (Hur.) 
