126 MAAZTLAN BIVALVES 
is the pattern constant, though there is a general adherence to 
a particular plan. Jt often presents the appearance of grains 
of detritus cemented together by animal matter, these grains 
also covering the body of the shell much more coarsely than 
in L. aristatus. It is thus that the young shells can generally 
be separated ; the epidermal tubercles being much larger, and 
the concentric wrinkles stronger. The number of specimens 
however was not large enough to ascertain these points with 
precision. The differences in the pattern of incrustation do 
not accompany those in form. The shape presents the same 
two extreme varieties, gracilior and tumidior, as in L. aristatus. 
The parallelism of the margins also is not constant. The bur- 
rows are in most respects like those of L. aristatus, g. v. They 
are shaped like the shell, and allow very little space for the 
opening of the valves. The orifice is almost close to the ex- 
terior, and is not bilobed. ‘The largest perfect specimen 
measures long. 1°86, lat. °53, alt.°5. A very tumid sp. in 
Mr. Darbishire’s Col. measures long. 1°9, lat. °9, alt. °6. 
Hab.—Panama, in Spondyli, Cuming.—Philippines [?], Han- 
ley.—Mazatlan ; rare in Spondyhi, very rare in Chame and 
Patella Mexicana ; L’pool § Havre Col. 
Tablet 572 contains 6 pairs young, the smallest ‘08 in 
length.—573, 4 sp. adolescent and adult, typical form.—574, a 
sp. In situ, burrowing in the attached part of the Spondylus 
calcifer: portions of the rock remain, which appear to have 
been bored by Pholads; a burrow is seen of P. calva.—d75, 
2 fragments shewing the extremities of the burrows which are 
more nearly round than in L. aristatus.—576, fragments illus- 
trating shell structure.—577, do. incrustations. 
Tablet 578 contains 3 sp. different ages, var. gracilior. 
Tablet 579 contains 1 pair and 2 valves do., var. twmidior. 
Tablet 580 contains a sp. with the anterior part less swollen, 
and the lines not parallel. 
176. LirhopHagus Aristatus, Sol. 
Mytilus aristatus, Dillw. Deser. Cat. 1817, vol. i. p. 303, no. 8.— 
Solander, ms..— Wood, Ind. Test. pl. 12, f. 8. 
Lithodomus aristatus, Forbes & Hanl. Br. Moll. vol. ii. p. 212. 
Enc. Méth. Vers, pl. 221, f. 8, a, b. 
Le Ropan, Adans. Sen. p. 267, pl. 19, f. 2. 
Mytilus lithophagus striatus, J. Sow. Linn. Trans. 1804, 
vol. viii. p: 274, pl. 6, f..2, 3-5. 
