MAZATLAN BIVALVES 11 
lapping horny lamine over the posterior portion. The snout 
is nearly round outside, displaying the gaping ends of the valves 
within, and a bi-partite lamina, rather rugose, between the two. 
The margin is entirely closed (except at the anterior part) with 
thick horny epidermis, strengthened ventrally by along narrow 
plate, not only filling up the gape of the young shell but 
projecting beyond the knob (now absorbed) into the anterior 
portion ; smooth, with a faint line in the middle: and dorsally 
by a thicker plate, with rather rounded projecting edges more 
or less swollen towards the umbos. These, with about a third 
of the surface of the valves, are covered with an irregularly 
shaped shield, smooth externally, encircling part of the dorsal 
plate, and with an internal, bi-partite, anterior lamina, fitting 
the valves. Between this shield and the hinge there is vacant 
room, but there appears no place of exit for the foot except 
at avery slight anterior chink, which seem to be all that the 
stretching power of the integuments can allow. The markings 
within vary considerably, the thick anterior ridge being often 
quite absorbed, and the hinge apophyses irregular. 
See a very full account of the animals of British Pholadide, 
and the relations between lamellata and papyracea of Turton, 
in Clark’s Moll. Test. Mar. Brit. p. 169—212. 
The following are measures of different specimens :—* 
Largest (acuminate) Long. °99 Lat. 1°93 Alt. .98 
Shorter form * "81 1°45 “78 
Smallest adult 95) 67 34, 
Smallest jun. "04, 06 P04 
Hab.—Adult in Spondyli, 12 fm., Isle of Perico in Panama 
Bay ; jun. in hard stones at low water, zd. ;+ Cuming.—Mazat- 
lan: abundant in Spondylus Lamarckii: also in ?primitive 
(not limestone) rock to which the Spondyli are attached : 
2 young specimens in Chame, and 1 in Strombus galea: 
Ppool § Havre Coll. ? 
Tablet 23 contains 8 valves very young.—24, 9 young speci- 
mens varying in size and acumination.—25, a young specimen 
with a part of its own burrow, in the tooth of a Spondylus, 
* In this family, and in Gastrochenide, the length was measured straight 
across, not to the middle of the ventral margin. 
t This is the accurate statement in the Proc. Z. 8.: in the Thes. Conch., it is 
given as ‘‘collected by Mr, Cuming in very hard stone at low water in Panama 
Bay ;” making it appear that all the specimens were thus collected, whereas the 
adults were found in Spondyli, 12 fm. deep. Similarly careless transcripts are not 
uncommon in the Monographs, 
