A Study in Variation . 
59 
as follows : ‘ shell very variable in shape, transversely ovate 
or oblato — deltoidal ; wider than long ; dorsal valve gibbous, 
most elevated near the front, evenly convex at the umbone ; 
mesial fold large, and more or less prominent. Ventral valve 
less convex than the opposite one, with a sinus of moderate 
depth, commencing at a short distance from the beak, and 
Pugnax cf. sulcatus J. de G. Sow. 
Fig. 2. A series of four specimens of each species to show the variation. The 
younger specimens are placed to the right. 
extending to the front. Beak small, much incurved, and but 
rarely visible in full grown shells. Each valve is ornamented 
with from nine to fourteen ribs, which become obsolete as 
they approach the beak and umbone ; from three to six 
occupy the fold and sinus/ 
Three examples measured : 
Length 12 lines. Width 17 lines. Depth 12 lines. 
,, 24 ,, ,,24 , . ,, 20 
,, 11 ,, ,, 15 ,, , ioj ,, 
ig33 March 1 
