MAZATLAN BIVALVES 157 
stages. The prismatic external layer, in rather large cells, is 
often beautifully apparent at this period. The naturalist has 
to rely on a balance of characters and the general habit of 
growth ; and even in these is hable to great error, unless he 
judge from a comparison of large series of specimens. Those 
from Mazatlan were numerous enough to confuse, not to help ; 
and therefore the following descriptions will need verification. 
211. OsTREA IRIDESCENS, Gray. 
Gray, ms. B. M.—Hanl. Conch. Misc. Ostrea, pl. 2, f. 6, 7. 
Ostrea sp. ind. b, C. B. Ad. Pan. Shells, p. 245, no. 381. 
Compare QO. prismatica, Gray, Ann. Phil. vol. xxv. 1826. 
Comp. O. spathulata, Zam. An. s. Vert. vol. vii. p. 225, no. 16. 
Comp. jun. O. margaritacea, Zam. loc. cit. p. 228, no. 26. 
Comp. jun. O. Aiquatorialis, D’ Orb. B. M. Cat. Moll. p. 88, 
no. 776:—?+(teste Gray) O. Puelchana, D’Orb. loe. cit. 
p. 87, no. 775. 
Jun. ?=Ostrea rufa (pars), Gould ms. (California.) 
As it is very doubtful whether the Mazatlan shells belong 
to either of the imperfectly characterized Lamarckian species, 
and as they certainly belong to the O. iridescens of Gray, 
just published by Hanley, I have adopted the latter name. It 
is recognized pretty distinctly (for an oyster) by its prolonged 
rectangular shape, long square hinge, laminated not undulated 
structure, and above all by the brilliant nacre, and rich brown- 
ish-purple, metallic lustre of the interior. A few flattened 
knobs appear on each side of the hinge in one valve, fitting 
into corresponding depressions in the other. Muscular scar 
large, reniform, variable. This sheil has long been known 
from a large series of very fine African specimens in the Bristol 
Museum, now, alas, for the most part lost. A valve which 
came into my possession in the year 1836 contained abundance 
of Lithophagus aristatus, and 2 valves of Placunanomia per- 
noides: I believe also that valves of Petricola robusta were 
out of the same oysters, as there was at that time scarcely a 
single West Coast shell in that Museum ; but of this I am not 
certain. On finding the same Oyster and the same Placuna- 
nomia, along with the same Lithophagus, in the Mazatlan 
collection, I sought for further confirmation of so singular a 
fact in geographical distribution. I therefore not only referred 
to the Bristol Mus. Cat., in which the locality was entered as 
“West Africa” in Mr. 8S. Stutchbury’s own hand; the em- 
ployés at the Institution bearing testimony to the same fact ; 
Mareh 1856. . » 
