ACEPHALA. 387 
behind the centre; the surface is very irregular, with coarse, concen- 
tric waves ; cardinal tooth small and conical ; ligamentary callus stout, 
erect; pallial sinus not deep, but broadly rounded ; posterior muscular 
impression well marked ; cavity of beaks profound. 
Length two and a half inches; breadth two inches; height an inch 
and a half. 
Obtained at the mouth of the Rio Negro, Patagonia. 
Besides being destitute of ribs, it is more nearly equilateral than P. 
arctica, and has the beaks removed farther back, and is less gaping ; 
posterior truncation scarcely oblique. 
Figure 499, lateral view of the shell. 
SoLeEN sicartus (Gould). 
T. transversa, oblonga, retrorsum angustata, sub-falcata, epidermide 
crasso, nitido, corneo induta, anticé oblique truncata, posticé rotun- 
data: valvis valdé excavatis, undulatis, in arets triangularibus indis- 
tincte partitis ; margine dorsal recto ; margine ventrali arcuato ; natt- 
bus terminalibus : cardo dente triangulart, erecto, recurvo in utrdque 
valva instructus. 
Solen sicarius, Goutp; Proceed. Boston Soc. Nat. Hist., iii. 214. 
May 1850. Expedition Shells, 74. 
SHELL elongated, transversely oblong, subcylindrical or tubular, 
somewhat falcate ; beaks terminal; anterior extremity truncate obliquely 
at an angle of about 30°, somewhat everted, the portion posterior to a 
line across from the beak to the base, concave; posterior extremity 
rounded; dorsal edge rectilinear; ventral edge regularly arcuate, so that 
the posterior extremity becomes about two-thirds the height of the 
anterior ; surface notably undulated by the stages of growth, parallel 
to the posterior end; the dorsal and ventral triangles not definitely 
marked, covered by a firm, glistening, horn-coloured epidermis ; valves 
very convex. Hinge with a single, erect, recurved, triangular tooth 
in each valve. 
Length three and a half inches; breadth seven-twentieths of an 
inch; height nine-tenths of an inch. 
