3IA2ATLAN 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 liable to great error, unless he 

 judge from a comparison of large series of specimens. Those 

 from Mazatlau were mmierous enough to confuse, not to help ; 

 and therefore the following descriptions wUl need verification. 



211. OsTEEA lEiDEScENS, Gray. 



Gray, ms. B. 'M.Sanl. Conch. Misc. Ostrea, pi. 2, f. 6, 7. 

 Ostreasp. ind. h, C. B. Ad. Pan. Shells, i). 245, no. 381. 

 Compare O. i^rismatica, Gray, Ann. Phil. vol. xxv. 1825. 

 Comp. O. spatliulata, Lam. An. s.- Vert. vol. vii. p. 225, no. 16. 

 Comp. jun. O. margaritacea, Lam. loc. cit. p. 228, no. 26. 

 Comp. jun. O. iEquatoriahs, D'Orh. B. M. Cat. Moll. p. 88, 



no. 776 :—? + (teste Gray) O. Puelchana, D'Orh. loc. 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 cejtainly belong to the O. iridescens of Gray, 

 just published by Ilanley, I have adopted the latter name. It 

 is recognized pretty distinctly (for an oyster) by its prolonged 

 rectangular shape, long square hinge, laminated not xmdulated 

 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 shell 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- 

 uoides : 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 

 aertain. 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. S. Stutchbury's own hand ; the em- 

 ployes at the Institution bearing testimony to the same fact ; 

 March 1856. p 



