8 MAZATLAN BIVALVES 
Tablet 18 contains 8 valves, very young, in test tube.—Tablet 
19 contains 4 adult valyes.—Tablet 20 contains Ostrea Virginica, 
‘with Chama ? spinosa attached, and 3 fine specimens of D. 
Cumingii in situ. One is removed from its nestling place be- 
tween the Chama and Oyster, and laid open to shew the inside. 
CLASS LAMELLIBRANCHIATA, Blainv. 
Conchifera: Lam. Phil. Zool. 1809; Ans. Vert. vy. 411, ed. 2, 
vi. 1.—Gray, Syn. B. M. 1840, 1842, p. 72; Proc. Zool. Soe. 
1848, p. 183. 
Acephala Lamellibranchiata: Rang, Man. Moll. 272.—Forbes 
and Hanl. Br. Mol. i.55.—Clark, Moll. Test. Mar. Br. 23. 
Famity PHOLADIDA. 
Pholadide, Gray, P. Z. 8. 
Genus PHOLADIDEA, Twrt. 
Pholas, Lin. Lam. 
15. PHoLADIDEA MELANURA, Sow. 
Pholas melanura, Proc. Zool. Soc. 1834, p. 70.—Sow. Thes. 
Conch. 1849, p. 499.— B. M. Cat. D’ Orb. Shells, no. 479, p. 56. 
This belongs to the same group of species as tridens, quadra, 
spathatula, and concamerata, which last represents it on the 
Californian coast. In its posterior cup-like appendages, it re- 
sembles Talona clausa from the Gambia. The shell is extremely 
thin, shewing inside a row of oblong tubercles impressed from 
without on the line separating the anterior and posterior por- 
tions, as well as the anterior ridges. These are closely set, 
strong, and waving; and passing over the medial depression 
at an angle slightly widening as the shell becomes adult, are 
continued, concentric and straight, ‘gradually fading as they 
approach the posterior extremity. This is covered with a thick 
epidermis, ending in a short double cup, which is bi-lobed 
outside and simple within. The adult closing of the valves is 
carried in a flat expansion round to the umbos. The largest 
specimen measures, with the cup, long. ‘9, lat. 1°85, alt. 8°6. 
Hab.—In hard clay and loose stones at low water, Monte Christi 
(Guayaquil), Cuming.—Ecuador, Monte Christi, D’ Orbigny. 
—Mazatlan; in company with P. acuminata, extremely 
rare ; L’pool Col. 
Tablet 21 contains the most characteristic (though a some- 
what imperfect) specimen. 
