ON AN UNNAMED SPECIES OP PECTEN. 
Description. — Valve triangularly orbicular, nearly equilateral, 
the antero-ventral border more sharply curved than tho posterior; 
surface with ten major folds, subacute, with a summit ridge and 
generally two lateral ridges on either side ; interspaces occupied by 
from 2-6 costulse, the central usually stronger. All the riblets are 
closely lamellose or tegulate. Ground surface finely granulate, 
with shagreen texture, tending to develop into transverse 
undulate ornament at the extremities of the valves. Ears very 
unequal ; anterior triangular, with outer margin truncated, having 
five radial cost® ; interspaces granular ; posterior triangular, 
truncated ; faintly costate and granular. 
Measurements of type — Height, 25 mm. ; length, 24 mm. 
Remarks . — This form differs from P. consobrinus in having 
normally ten folds instead of eight. The folds are subacute, whereas 
in P. consobrinus they are gently convex and equally and numerously 
costate. In P. praecursor, moreover, the transverse ornament of 
the riblets is, in fresh specimens, more decidedly tegulate, whereas 
in P. consobrinus the ornament is a series of delicate concentric 
frills. 
This species appears to pass upward into P. consobrinus (Kalim- 
nan) by loss of major folds, and to pass downward into P. foul- 
cheri (Janjukian) and Balcombian by increase of folds and transition 
of tegulate ornament into the erect squamose. 
