PINNA. 257 



(about a third or a fourth of the entire area) is not 

 traversed by them. No echination is visible, for the most 

 part, upon the more aged examples, but short upright 

 caducous vaulted scales, of small size, and disposed at 

 moderately distant intervals, often crown the ridges of the 

 younger specimens ; in which case there is generally a 

 kind of obscure radiation of obsolete scales upon the portion 

 otherwise devoid of sculpture. The concentric wrinkles of 

 increase, although not forming (as in certain PinntB) a 

 distinct lattice- work, are nevertheless rather prominent in 

 the older shells, run nearly at right angles to the dorsal 

 or hinge margin, and often give, by their convergence at 

 the basal margin, a laminated appearance to the structure 

 of the valves. The dorsal edge is not (as in certain allied 

 species) incurved towards the beaks, but is nearly recti- 

 linear in the more regularly developed individuals, with a 

 descending inclination near the narrow end of the shell. 

 The ventral edge, after the ordinary anterior indentation and 

 subsequent swell, runs in young shells (that do not exceed 

 six inches in length) almost subparallel to the upper margin 

 and in nearly a straight line, so that (except in stunted 

 individuals) the broad end of the valves is somewhat 

 squared, and by no means peculiarly broad ; the terminal 

 edge is, however, convex or even arcuated. With advanc- 

 ing age, as the upper and lower margins diverge farther 

 from each other, the general contour becomes more broadly 

 triangular, the dorsal edge slopes a little downward at its 

 termination, the ventral margin becomes more produced in 

 proportion (in the young it is inferior in length to the 

 opposite edge), and the terminal or posterior outline usually 

 assumes a more rounded or arcuated appearance. The 

 interior either partakes of the external colouring, or is dark 

 and subnacreous. 



VOL. II. L li 



