ACEPHALA. 421 
Venus rigida, Goutp; Proceed. Boston Soc. Nat. Hist., iii. 277. 
July 1850. Expedition Shells, 85. 
SHELL large, inequilateral, transversely ovate, ashy white, more or 
less compressed ; beaks near the anterior third, pointed, antrorse, with 
an indistinct, broad, lanceolate areola in front; anterior side two-thirds 
the height of the posterior, rounded at the end, dorsal margin com- 
pressed ; posterior side a little gaping, slightly truncate at end, dorsal 
edge broad, nearly rectilinear, and horizontal; beaks smooth or deli- 
cately striate: surface grooved with radiating lines, much the deepest 
and most distant posteriorly, decussated with concentric lamine, about 
equally distant, reflex especially at the anterior end, where they are 
most developed ; on the posterior umbonal slope many of the concen- 
tric ridges disappear; the decussating points anteriorly are deeply 
pitted. Interior dull porcelain-white ; ventral margin simple or finely 
crenulate in old specimens. Hinge strong, with three teeth in each 
valve, the central one in both valves, and the posterior one in the right 
valve, bifid. Siphonal sinus lanceolate, reaching to nearly under the 
beaks. 
In one specimen, length three and three-fourths inches; height two 
and three-fourths inches; breadth one inch and one-half. Of another, 
leneth two inches; height one and five-eighths inches; breadth one 
inch and one-fifth. 
Inhabits Oregon, Puget Sound, and Straits of De Fuca. Mr. Case. 
Dr. Pickering. 
Has the form and general aspect of Sazidomus Nuttalli, Conrad, in 
which the hinge teeth are quite different, being more divergent, the 
central tooth being actually divided, and having moreover a supple- 
mentary one in front; and there are no traces of diverging lines. ‘The 
sculpture is much finer than in V. Dombeyana, and more like that of 
V. decussata. It varies considerably in its size and proportions, being 
often much more expanded. 
Figure 588, side of a dilated specimen; 538 a, side of a medium 
specimen ; 538 8, dorsal view of the latter. 
106 
