I MAZATLAN BIVALVES 
shewing the grey lining and part of the pedal chink.—26, the 
adolescent specimen.—27, 6 adult specimens shewing various 
peculiarities of growth.—28, a small adult specimen in situ, 
with a thick irregular lining: another specimen remains en- 
tombed.—29, an adult with part of its burrow, shewing the 
stratified nature of the lining: the anterior portion of the shield 
curiously deformed.—30, a piece of Spondylus, with 3 young 
specimens in situ, and several burrows, shewing the stratified 
lining, and the pedal excavations reaching the inside of the shell, 
to the evident annoyance of the Spondylus, which has protected 
itself against one of its enemies by a protuberance ‘7 across, 
and *23 high.—31, a large piece of Spondylus with various 
burrows: a large one of calva, with enormous foot chink: a 
smaller one in the hinge tooth: two others with small foot 
marks: a burrow of ?Lithodomus encased from an old hole 
into which it had penetrated: a singularly twisted burrow of 
Gastrochena, bent nearly double, &c.—32, fragments of the 
horny ?foot.—33, fragments of the grey deposit.—34, fragments 
shewing hinge structure.—35, fragments illustrating the cup- 
laminze.—36, portions of the umbonal plate.—37, portions of the 
dorsal and ventral plates. 
18. PARAPHOLAS ACUMINATA, Sow. 
Pholas acuminata, Proc. Zool. Soc. 1834, p. 70.—Sow. Thes. 
Conch. 1849, p. 492. 
The author of this species distinguishes it from calva by the 
shape of the lamine and posterior portion, which are variable 
in both forms; and by the character of the umbonal shield. 
This last is the only constant character of difference. It is not 
only smaller, not projecting beyond the dorsal plate ; (which is 
not the result of age, being found in all the specimens ;) but, in 
all the specimens allowing of observation, it is turned-in all 
round, instead of at the anterior portion only as in calva. The 
external surface also is generally rougher, and the posterior 
gape smaller, not displaying the bipartite lamine so clearly. 
Still, as the shells exactly agree in all other respects, it is pro- 
bable that these differences only result from changes in situation. 
All the calvee were taken out of Spondylus: all the acumi- 
nate were sent loose; and from their extremely perfect con- 
dition were probably extracted from clay or wood. If the latter, 
the irregularities of the decaying timber might cause the 
roughening of the plate-surface. The original specimens of 
acuminata however were taken out of argillaceous limestone. 
The largest specimen measures long. °8, lat. 1°54, alt. 77. 
