242 TESTACEOUS MOLLUSCA. 
Ovate-elliptical, thin, convex, posteriorly with a few radiating rib- 
like striae, elsewhere smooth and glossy ; the anterior surface raised 
and dilated, its end rounded; cuticle brownish, or yellowish. 2..1. 
North Pole—General shape of Impactus. 
M.Discrepans* Gould Mas. p. 129—Turt Biv.—Myrtiwus D. 
Mont. p. 169—D. p. 319—W. t. 12. f. 38. Oval-oblong, com- 
pressed, greenish fulvous, becoming green at the edges, thin; an- 
terior area marked with minute radiating simple strize and elevated 
as it were above the smooth central surface; posterior end with a 
few rather distant radiating grooves, obtuse; dorsal line elevated 
and arcuated ; ventral edge with a distinct sinus at the commence- 
ment of the anterior area,and convex in the middle of the central 
2 
area. 2.—WN. Seas. 
M. Nicra.* Brit. Mar. Con—Myrtitus Discrerans. Mont. Sup 
t. 26. f.4.—Son. G. f. 3. (good )— Reeve Syst. t. 100. f- 3.—Ch. f- 
767.—M. Discors var Suecica Otho. Fab. Dankse V. S. Sk. 3. 
(1788.) Oblong or elongated-oblong, arched, rather thin, convex, 
with a blackish cuticle becoming tawny towards the edges ; anterior 
half of the shell not elevated but covered with numerous raised 
subcrenulated sulci, the central area smooth and the posterior end 
with several radiating costellee, and rather prominent ; hinge margin 
but slightly elevated, much arcuated ; ventral edge incurved in the 
middle, dilated anteriorly ; beaks obtuse. 14.— Sweden § Scotland ! 
M. Fasa.—Myrtitus F. Gmel. 3359.—Ch. f. 761.—W. t. 12. f. 
41.—D. p. 320.—Mop. Arctica. Leach in Ross Voy. Roundish- 
oval, ventricose, white under a reddish or olive cuticle, with rather 
distant radiating strize : inside pearly, the ventral edge rounded and 
crenated. 3.—N. Seas. 
M. Pectinuta Gould Mass. f. 85. Small, ovate, ventricose, 
with about forty equal rounded radiating ribs : epidermis dark gam- 
boge yellow ; beaks prominent, projecting as far as the posterior 
margin : within livid or leaden pearly. 3..=5.—N. America. 
M. Nexa. Gould. Mass. p. 128. f. 86. Small, ovate, thin, rusty 
brown with shades of olive, sculptured by a network of very mi- 
nute crowded lines of growth and very numerous fine indented 
radiating lines which are obsolete in the middle and strongest ante- 
riorly, where a very fine line divides each into two; while above 
these on the compressed portion a beautifnl network of hexagonal 
indentations is formed: beaks prominent and placed considerably 
before the posterior end: nacre livid. =. qo U. States. 

1 | observed in the Museum at Paris under the name of Drrressa 
a species from Newfoundland, closely allied to this, but quadruple 
the size, less narrow, the ventral edge scarcely incurved, and the 
radiating striz distinctly crenulated and infinitely more crowded 
and delicate. 
