93 
angles to axis of shell, and a minute tubercle near the beak. 
Dorsal valve very concave. Area wide, formed equally from 
both valves, turned over into the plane of the shell, slightly 
hollowed. Triangular foramen occupied by dental process. 
Ribs 10 or 11 around the margin, stout, convex, simple or with 
two or three bifurcated ones. Concentric striae sometimes con¬ 
spicuous. 
Resembles C. gibbosa Hall and C. Koninckana H. & P. in 
the direction of its spines, but differs from these and related 
species in the small number of its ribs. 
Strophodoixta erratic a. —Resembles 8. ctrcuata Hall in 
hinge structure, but central cardinal process of ventral valve 
is narrower, and dental lamellae denticulated. Divaricator 
scars drawn to a point on median line; hinge-line abruptly 
acuminate. Ribs few and large on the umbo, increasing by 
implantation, and diminishing in size toward the margin. In 
variety solidicosta, about 9 large nearly undivided ribs; in 
variety fissicosta , ribs fimbriated till they number 50 to 80 
around the margin. The last variety, except in the ears, re¬ 
sembles 8. subdemissa Hall. Comp, also with 8. plicata Hall, 
(XIII Reg. Rep., p, 90) and S? costata Owen, (Surv. Wis, 
Minn. & Io., Tab. Ill, Fig. 11). 
Strophodonta imitata. —Adductor bosses prominent, bi- 
crescentic, with a short, stout, median ridge issuing from 
between them forwards. Otherwise resembles S. incequistri- 
ata , except that the striae are nearly equal, and the shell is 
relatively shorter and only 12.7 mm. (.5) to 17.8 mm. (.7) broad. 
Strophodoivta crucTA. —Size and general appearance of 
8. incequistriata. Hinge-line less than greatest width. hinge- 
angles rounded. Inside of ventral valve minutely pustulose in 
all parts, and marked by a prominent ridge all around near the 
border. Divaricator scars diverging, reaching two-thirds the 
distance to the anterior border; retractor scars nearly as long. 
Spirifera bidorsalis. —Resembles 8. biplicata and bime- 
sialis , but is not produced at hinge extremities; has a high 
incurved ventral beak, and delicate, regular imbricating 1am- 
ellas. Length 8.88 mm. (.35) ; breadth 12.7 mm. (.5). Very 
commonly parasitic on corals. 
Spirifera consors. —Semicircular, with salient hinge ex¬ 
tremities, ventricose to the margin. Dorsal valve with little 
elevated but strongly isolated fold, having a median furrow 
throughout its whole length; ventral, most ventricose, es¬ 
pecially near the incurved beak; having a broad, sharply- 
rounded, well-defined sinus reaching to the beak and destitute 
of a median ridge. Dorsal area narrow; ventral elevated, 
arched and perforated by a triangular opening half as broad 
as high. Surface with about seven rounded plications each 
side of the middle, and crossed by feeble lines of growth. 
