12 MAZATLA.N BIVALVES 



shewing tlie gi'ey lining and part of tlie pedal cldnk.— 26, the 

 adolescent specimen— 27, 6 adult specimens slievdng various 

 peculiarities of srowtli.— 28, a small adult specunen in situ, 

 Mith a thick irregular lining : another specimen remams en- 

 tombed— 29, an adult with part of its buiTOw, shewing the 

 sti-atified nature of the lining : the anterior portion of the shield 

 curiously deformed— 30, a piece of Spondylus, with 3 young 

 spechnens in situ, and several bui-rows, shewmg the stratihed- 

 lining, and the pedal excavations reaching the mside of the shell, 

 to the evident annoyance of the Spondylus, which has protected 

 itself ao-ainst 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 chmk : a 

 smaUer one in the hinge tooth ; two others with smaU foot 

 marks • a burrow of PLithodomus encased from an old hok 

 into which it had penetrated : a singularly t\visted burrow of 

 Gastrochfcna, bent nearly double, &C.-32, fragments of the 

 hornv Pfoot— 33, fragments of the grey deposit— 34, fragments 

 shewing hinge structure.— 35, fragments illustratmg the cup- 

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

 dorsal and ventral plates. 



18. Paeapholas acuminata, Soto. 

 Pholas acuminata, Proc. Zool Soc. 1834, p. 70.-^ow. Thes. 



Conch. 1849, p. 492. 



The author of this species distinguishes it from calva by the 

 shape of the lamins and posterior portion, which are ;<-anable 

 in both forms; and by the character of the umbonal shield 

 This last is the only constant character of difference. It is not 

 only smaUer. not projecting beyond the dorsal plate ; (which is 

 not the result of age, being found in aU the specimens ;) but, in 

 all the specimens allowing of observation, it is tnrned-m aU 

 round, instead of at the anterior portion only as in calva. lUe 

 external surface also is generally rougher, and the posterior 

 <rapc smaller, not displaying the bipartite laminae 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 calva^ were taken out of Spondylus -. aU the acumi- 

 uata. were sent loose ; and from their extremely Pp^^f^'^t con- 

 dition were probably extracted from clay or wood. U the latter, 

 the irregidarities of the decaying timber might cause the 

 rou-hening of the plate-surface. The origmal specunens of 

 acuminata however were taken out of argillaceous limestone. 

 The largest specimen measures long. S, lat. 1 54, alt. u. 



