162 MAZATLAN BIVALVES 
Africa ;) from which it is known by the minute denticles on 
each side of the hinge. One valve, 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 avery 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 umbos. It begins 
life as aswollen regularly formed body, of the shape of Astarte 
compressa. A specimen from 8. W. Mexico, in a dead Cyprea, 
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, and, 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 
erenated 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. 
Hab.—Mazatlan ; not uncommon, on various shells, v. infra ; 
*pool Col.—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; Nuttal/—? W. Africa, in dead Pinna rudis, 
PP..0: 
Tablet 715 contains 1 pair and 7 valves, extremely young, 
the smallest °04 across, chiefly from Chame and Spondyl.— 
716, asp. of Anomia lampe, with extremely young oysters in 
situu—717, 4 valves, a stage older.—718, a young sp. opened, 
