8s MAZATLAN BIVALVES 
is strong, curling round the umbos, like Isocardia, and becom- 
ing semi-internal beyond the hinge teeth. All the specimens 
are dextral. The lamine are generally finely striated outside. 
These shells, like those of Spondylus, offered a safe retreat 
for boring and nestling bivalves and numerous minute Gas- 
teropods. The smallest valve measures ‘02 across. That with 
the largest animal, (shell somewhat thin,) long. 4°5, lat. 4°25. 
alt. 3'4. The heaviest shell measures externally {though part 
has been removed) long. 5°7, lat. 5°, alt. 5°; internally (from 
ligament to margin) long. 2°6, lat. 2°8, alt.2°2. This, with 
another smaller specimen attached, weighs 5lb. ; and displays 
a very large and persistent spiral ligamental groove. 
Hab.—Guif of Tehuantepec, Mexico; dredged up from sandy 
mud attached to Avicule, 10 fm. ; Mus. Cuming.—Mazatlan ; 
not cominon; L’pool & Havre Coll.* 
Tablet 425 contains 3 valves, extremely young, exhibiting 
the shagreened surface at the umbos.—426, 2 do. umbos finely 
striated.—427, 1 do. umbo spiral, as in young Calyptreade.— 
428, 1 do. concentric foliations developed. — 429, 8 valves, a 
stage older, spiny processes developed, valves inside variously 
coloured, and surface rugose.—430, 1 pair and 3 valves, a stage 
older, colour highly developed. 
Tablet 431 contains 1 pair young.—432, a specimen with the 
spiral umbos so enormously developed, as to approach the fos- 
sil form Diceras. The umbo of the attached valve makes two 
loosely spiral whirls, the hgament running up the suture; in 
length it nearly equals the breadth of the shell, but being 
filled up with solid matter, would not display itself in the 
internal cast. 
Tablet 433 contains a pair, young, purple and orange, (closely 
resembling the C. echinata, “‘very old,” figured in Beechey’s 
Voyage.) Margin very finely, and teeth strongly crenate. 
Outside, with Vermetide, Lithophagi, &e.—434, 2 valves at- 
tached to each other, pierced by Lithophagi, &c. The outer 
layer has become eroded, except where the Lithophagi have 
bored. 
Tablet 435 contains a diseased specimen, purple, attacked by 
Lithophagi, even to the very centre of the teeth. 
Tablet 436 contains a finely grown adult specimen, attached 
to Pinna, with Vermetide &c.—437, the sp. with the largest 
animal.—438, the group with the largest shells. 
* A specimen received from Dr. Gould of this species (without reference) is 
probably the ‘OC. Pacifica” of his list, ‘‘Guaymas, Lieut. Green.” It may 
however be the C, Buddiana, C. B. Ad.; v. infra. 
