162 MAZATLAN BIVALVES 



Africa ;) from which it is known by the minute denticles on 

 each side of the hinge. One T alve, of a rich orange within, 

 has a deep purple streak outside gradually shading off into the 

 most delicate penciling, and another faint ray of clouded 

 purple, leaving two orange areas ; the purple bordering the 

 inner margin, and shading most exquisitely into the orange. 

 Oysters are rarely seen of such surpassing beauty. Ordinarily 

 it is a very small purple shell, frequenting other shells even 

 when living, and often interfering with their growth. It is 

 known by the very small triangular ligament area, with very- 

 fine denticles on each side. In addition to these, there are 

 generally (in the attached valve) two lines of denticles situated 

 in furrows running outside the area to the urnbos. It begins 

 life as a swollen regularly formed body, of the shape of Astarte 

 compressa. A specimen from S. W. Mexico, in a dead Cyprsea, 

 retains this regularity and general form : it is smooth, white, 

 and convex. The hinge of the attached valve, when extremely 

 young, reminds one of Mesodesma ; the ligament being internal 

 between two raised processes followed by pits ; the processes 

 afterwards developing into the ligamental area. Sometimes 

 the shell becomes rather thick, arid, after continuing smooth 

 and flat, suddenly rises, and waves (almost plicating) the outer 

 margin. The young of the stronger shells being brilliant in 

 colour, might easily be taken for those of O. iridescens : if 

 however, (as I have supposed from the valves already quoted) 

 the young of that shell is not crenated, the denticles afford a 

 good distinguishing character ; and, at a later period, the 

 crenated sulei round the area. I have specimens received as 

 from W. Africa, at any rate inhabiting an Atlantic Pinna, 

 which present the same colouring, habit of growth, and some- 

 what remarkable hinge. They seem from the crenations not 

 to be the young of O. bicolor. The finest grown specimen 

 measures long. 1'04, lat. '94, alt. '12. 



Hob. Mazatlan ; not uncommon, on various shells, v. infra ; 

 L'poolCol.S. W. Mexico, do., P.P. C.? Panama, at- 

 tached to stones, rocks and shells, near half-tide level, C. B. 

 Adams. San Diego, very fine ; Lieut. Green. San. Diego 

 and Oregon ; Nuttall. ? W. Africa, in dead Pinna rudis, 

 P. P. C. 



Tablet 715 contains 1 pair and 7 valves, extremely young, 

 the smallest "04 across, chiefly from Chamse and Spondyli. 

 716, a sp. of Anomia lampe, with extremely young oysters in 

 situ. 717, 4 valves, a stage older. 718, a young sp. opened, 



