164 MAZATLAN BIVALVE8 



Tablet 735 contains a young elongated specimen. — 736, 

 another, rounded form, -with a few tubular spines near the mar- 

 gin of the upper valve. — 737, the largest sp. with Vermetidae, &c. 



215. OsTEEA , sp. ind. 



? = Ostrea sp. ind. e, C. B. Ad. Pan. Shells, p. 246, no. 384. 

 Ostrea iridescens, jun. P. P. C. Cat. Prov. 



Agreeing with O. iridescens in its square form, straight 

 hinge, and nacreous interior ; differing in the irregidar radia- 

 tion of the lower valve, and strong sharp plication of the mar- 

 gin. This differs from the undulated margin of O. c. palmida, 

 in not having a distinct limb, with remote line of denticles. 

 These, if present, are very small, and rather near the margin. 

 The characters are so variable that I should have continued to 

 regard this as the j'oung of O. iridescens, but for finding 

 specimens of the latter without hinge denticles, which in these 

 shells are strongly marked ; and for the remark of Prof. Adams, 

 with regard to what appears the same species, that the animal 

 has a bitter flavour. Long. 2 "5, lat. 2 '5, alt. 1'35. 



Hah. — ]\Iazatlan ; very rare, on shells ; Uiwol Col. — ? Pan- 

 ama ; very common, attached to rocks and stones from one 

 to thi-ee-quartcrs tide level : small var. on Nerita scabri- 

 costa ; C. B. Adams. — San Diego, Lieut. Crreen. 



Tablet 738 contains 7 very young valves, off Spondyli and 

 Chamse. — 739, 1 do. on Imi^erator olivaceus. — 740, 1 do. on 

 Imp. unguis, distorting the shell. — 741, 1 yoimg. white valve, 

 resembling O. Virgiuica, but with crenated hinge. — 742, 2 

 valves approaching O. iridescens. — 74.3, 4 paii-s strongly plicate, 

 green within. — 744, 1 large pair, attached by portion of the 

 valve to Pinna. 



Family A2s^0ML\DyE. 



Genus PLACUNANOMIA, Brod. 



Brod. in Proc. Zool. Soc. 1832, p. 2d.—Gra// in B. M. Cat. 



Anom. p. 8. 

 Subgenus INIonia, Gray, Proc. Zool. Soc. 1849, p. 121. 



216. Placunanomia peenoides, w. *. 



P. t.formA maxime variante, intei'dum transversd, interdum 

 maxime elongota, valvd inferiori maxime perforatA, cuneo intus 

 liris haud tcnuibus instructo, fidcro ligamenti valde extante, 



