12 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 sir all 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 PLithodomus encased from an old hole 

 into which it had penetrated : a singularly twisted burrow of 

 Gastrochsena, bent nearly double, &c. 32, fragments of the 

 horny Pfoot. 33, fragments of the grey deposit. 34, fragments 

 shewing hinge structure. 35, fragments illustrating the cup- 

 laminse. 36, portions of the umbonal plate. 37, portions of the 

 dorsal and ventral plates. 



- 18. PAEAPHOLAS ACUMINATA, Sow. 



Pholas acuminata, Proc. Zool. Soc. 1834, p. 70. Sow. TJies. 



Condi. 1849, p. 492. 



The author of this species distinguishes it from calva by the 

 shape of the lamina? 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 laminse 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 calvse were taken out of Spondylus : all the acumi- 

 natse 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. 



