LAMELLIBRANCHIATA. 61 



Pterinopecten exfoliatus. 



PLATE I, FIGS. 16, 17; AND PLATE LXXXIII, FIGS. B, 7. 



Pterinopecten exfoliatus, Hall. Pal. N. V., vol. v, pt. 1. Plat<>s and Explanations: PI. 1, figs. 16, 17. 



Jan., 18S3. 



Shell of less than medium size, sub-rhomboidal ; body obliquely ovate ; length 

 somewhat greater than the height ; pallial margins regularly rounded from 

 the anterior ear to the base of the posterior slope, from whence the margin 

 is nearly straight or slightly concave to the extremity of the hinge. 



Left valve very convex, inclined to gibbosity. Right valve moderately 

 convex, much smaller than the left. 



Hinge-line straight, a little longer than the greatest length of the shell, 

 extended posteriorly. 



Beak of left valve prominent, sub-acute, anterior to the middle of, and 

 arching over the hinge-line, directed forward. Umbo prominent, gibbous, 

 the greatest convexity being about the middle of the width, subtending a 

 right angle. 



Ears triangular. Posterior ear large, undefined ; margin slightly con- 

 cave ; extremity somewhat acute. Anterior ear small, convex, defined by a 

 deep, obtuse sulcus ; margin convex ; extremity acutely rounded. Byssal sinus 

 distinct, somewhat deep. 



Test thin ; marked in the left valve by fine, regular, radiating stria? with 

 fiat interspaces which show one or two finer rays ; crossed by concentric 

 lines of growth and stronger undulations, which in old shells become very 

 prominent. The rays are continued over the ears, and are somewhat sub- 

 dued and less distinctly alternating on the posterior ears, and obsolete over 

 the upper part of the umbo. The right valve is marked by obsolescent rays 

 and concentric undulations. 



Ligamental area narrow. 



Three specimens measure respectively as follows: Length 17, 20 and 22 

 mm., height 15, 18 and 18 mm., length of hinge-line 18, 21 and 23 mm. 



In the exfoliated condition, this species resembles P. insons ; but has a com- 

 paratively shorter hinge-line, smaller anterior ear, deeper byssal sinus, and less 



