MAZATLAN BIVALVES 



16. PHOLADIDEA PCURTA, Sow. 



Pholas curta, Proc. Zool. Soc. 1834, p. 71. B. M. Cat. D'Orb. 



Shells, no. 482, p. 56. 



The only two specimens found are too imperfect to identify 

 with accuracy.. Though very small, they are both adult, and 

 are known at once from the young of melanura by the extreme 

 fineness of the anterior waved striae, the posterior part scarcely 

 shewing more than lines of growth. Dorsal plate shield-shaped, 

 in two layers, hollow within, rather large and regular. Long. '28, 

 lot. '34; shield '17 by '15. 

 Hah. Isle of Lions, Yeragua, in soft stone at low water, Cum- 



ing. Ecuador : Isle de los Leones, \f\If QrMfrn/y. Mazatlan, 



in Strombus galea, Havre Col. 



Tablet 22 contains 1 valve (the other being broken in extrac- 

 tion) with its plate. 



GENUS PABAPHOLAS, Conr. 

 Parapholas, Conrad, sp. 



This genus, including Calif ornica, Incii (Torres Str.), branchi- 

 ata, calva, (acuminata,) and bisulcata (Woodw.) differs from 

 Martesia (Leach) in having its cup laminae persistent and under- 

 lapping one another. It further differs from Pholadidea in 

 the large size of the umbonal plate, and the gaping in the ven- 

 tral and dorsal margins, closed by plates in the adult shell. All 

 yet known are from the Pacific. The Californian species is of 

 large size, and makes a shelly tube for its siphons. 



17. PAEAPHOLAS CALVA, Gray, ms. 



Pholas calva Sow. in Proc. Zool. Soc. 1834, p. Q$Thes. Conck. 

 1849, p. 493. 



Animal excavating a pear-shaped burrow in shell (or stone) 

 which is perfectly smooth and circular transversely, so that the 

 shell [(till it becomes adult) can move freely round in it. The 

 orifice is subcircular, and rather large. As the animal continues 

 its boring deeper, the swollen anterior portion becoming now 

 posterior and therefore too large for the animal, which loves to 

 have just room enough and no more, the vacant space is filled 

 up with a lining (more or less thick according to the depth of 

 the burrow) which is not an organic growth from the mantle, 

 but appears to be nothing more than a sedimentary deposit 

 in layers. Whether the burrow is in the purple or white 

 portion of the Spondylus, the deposit is always dark grey. 

 It may be detached as a tube from the cavity, and is often per- 



