MAZATLAN BIVALVES 51 
white. The shape is very constant, only occasionally varying 
in the shortening of the anterior portion. The posterior lateral 
teeth are extremely small, the anterior of moderate size. Pal- 
lial sinus rather small and narrow.—I have seen this shell 
labeled as from the W. Indies, but without authority. It has 
probably been mistaken for Lutraria carinata, teste Gould, 
which he gives as the Caribbean analogue of Lutraria ventri- 
cosa, (=Mulinia ventricosa, C. B. Ad. Pan Shells, p. 293,) 
which is given from Panama and Mazatlan. ? Can this be the 
M. exoleta of Gray. V. Proc. Bost. Soc. N. H. iv. 89; awork 
which, alas! I have not been able to see. In the very young 
shell, the anterior margin is but little produced, and the pos- 
terior tooth is moderately large. 
Youngest specimen measures long. ‘16, lat. °21, alt. ‘1. 
The most ventricose _,, gas Oy ates Leet rig © LS: 
The largest * FEN ERA, pa abe i cht, Oe 
Hab.—Cape Horn [?] and West Columbia, Reeve.—Guayaquil, 
Hinds.—Mazatlan ; not uncommon, L’pool Col. 
Tablet 211 contains 1 very young valve.—212, 3 specimens ; 
one young, of exquisite beauty ; one with the epidermis re- 
moved, shewing the interior (presented by J. Hibbert, Esq.) ; 
the other very large. 
79. Mactra (Spisuna) FRAGILIS, Chemn. 
Conch Cab. vol. vi. p. 236, pl. 24, f. 235.—B. WM. Cat. D’ Orb. 
Moll. p. 56, no. 489.—B. M. Cat. Sagra Moll. p. 35, no. 418. 
=Mactra ovalina, Lam. An. s. Vert. ed. Desh. vi. 104, no. 21, 
(teste Gray.) 
=Mactra Braziliana, Lam. An. s. Vert. ed. Desh. vi. 106, 
no. 27, (teste Desh.) 
= Mactra oblonga, Say, (teste Rve.) 
Mr. Cuming having compared this shell critically with both 
larger and smaller specimens in his collection, writes that it 
certainly belongs to M. fragilis, Chemn. (according to Desh.) 
The texture and epidermis are very different from M. velata, 
Phil. of which, for geographical reasons, I had thought it 
might be the young. It is'‘remarkable for its double posterior 
ridge, great flatness, and large gape on each side. Long. 1°36, 
lat. 1°93, alt. °64. 
Hah.—W. Indies, Honduras and St. John’s, Mus. Cuming.— 
Cuba, Sagra.—Rio Janeiro, Lalande, jun. (teste Lam.)— 
