MAZATLAN BIVALVES 11 



lapping horny laminae 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 a long 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 a very 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 Pholadidae, 

 and the relations between lamellata and papyracea of Turton, 

 in Clark's Moll. Test. Mar. Brit. p. 169212. 



The following are measures of different specimens : * 

 Largest (acuminate) Long. '99 Lat. 1'93 Alt. .98 

 Shorter form '81 1'45 '78 



SmaUest adult "35 '67 "34 



SmaUestjun. '04 '06 ?*04 



Hal. Adult in Spondyli, 12 fm., Isle of Perico in Panama 

 Bay ; jun. in hard stones at low water, id. ;t Owning. Mazat- 

 lan : abundant in Spondylus Lamarckii : also in ? primitive 

 (not limestone) rock to which the Spondyli are attached ; 

 2 young specimens in Chamse, and 1 in Strombus galea : 

 L'pool fy 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 Gastrochsenidse/the length was measured straight 

 across, not to the middle of the central margin. 



t This is the accurate statement in the Proc. Z. S. : 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. 



