236 TESTACEOUS MOLLUSCA. 
narrow and seemingly curved upward, from the sudden obliquity 
of the centrally incurved ventral edge; anterior edge long, scarcely 
at all retuse, extremity produced, attenuated, rounded ; dorsal angle 
distinct, compressed ; within purple anteriorly. 11, . 24.—South 
America.? 
M. Sorpiwa. Nobis. Z. P. 1844. Oblong, ventricose covered 
with an olive green epidermis, and anteriorly with a lamellar mem- 
branaceous yellowish-ashy covering, which is here and there pro- 
duced into lanceolate spines; hinge margin rather short, but 
slightly elevated, the dorsal angle very obtuse; anterior edge 
elongated, (in the adult, retuse) extremity obtuse ; ventral edge 
strongly incurved ; umbonal ridge prominent, edged posteriorly 
with a paler ray ; posterior extremity extremely short and obtuse ; 
within tinged anteriorly with purple. 14..22.—General shape of 
M. Modiolus. 
M. Arcuatuta Nobis Z. P. 1844. Narrow, elongated, subar- 
cuated, compresssd, smooth, very fragile, under a greenish fulvous 
epidermis, marked on the anterior slope with transverse ziczac pur- 
plish-brown streaks ; the umbonal ridge elevated and whitish, often 
edged anteriorly with purplish brown; hinge-margin elongated, not 
elevated, no dorsal angle; anterior edge short, much arcuated, the 
extremity dilated and rounded ; ventral margin incurvated, posterior 
end prominent, rounded, much attenuated, with a SS radiating cos- 
tellaze ; within, tinged with purple autenionlys Z..14.—Philippines. 
M. Paracontca DOrbigny MSS. Oblong- anon the 
beaks being all but terminal and the posterior end narrewed and 
obtuse ; the anterior broad, obtuse and flat; hinge margin straight, 
somewhat elevated and elongated, the anterior edge in the adult in- 
stead of forming an angle, running at first almost in the same line 
with it, and then suddenly sloping in a semicircle to the nearly 
straight ventral margin; cuticle greenish olive ; interior tinged with 
purple, 1..2—Patagonia.! 
Section B. 
Smooth, oblong-subtrapeziform, the shape being intermediate between 
M. Modiolus and M, Lithodomus. 
M. Vacina Lam. 7.—E. vol. 2. p. 569.—Subrhombic-oblong, 
subcylindraceous, white under a uniform yellowish chesnut, cuticle 
very thin, pellucid, nearly smooth, marked only with close concen- 
tric wrinkles ; dorsal edge a little areuated, elongated, and sub- 
parallel to the ventral ; anterior extremity much rounded : posterior 

' To this section belong likewise the M. Castanea of Say in the 
second and. M. Cavax of Conrad in the seventh volume of the 
Journal A.N.S. of Philadelphia, Also the Pripzauxiu of Leach’s 
Zoological Miscellany. 
—— Ts 
