LAMELLIBRANCHIATA. 7 



AVICULOI'KCTKN SCABRIDUS. 



PLATE III. in.-. MS. 



Aviculopeiten scahriditu. Hall. Pal. N. V.. vol. \, pt. 1. Plates and Explanations: PI. :i. figs. :t 12 



Jan., 1883. 



Shell large, sub-circular, very slightly oblique to the hinge-line; length 

 somewhat greater than the height : pallia] margin circular, becoming verj 

 1'ul I posteriorly. 



Valves depressed, moderately convex. 



Hinge-line straight ; length equal to two-thirds the length of the shell, 

 marked by a thickening and inflection of the cardinal margins of the ears. 



Beaks obtuse, rising slightly above and arching over the hinge-line, 

 situated just anterior to the middle : umbonal angle 110°. 



Ears narrow, triangular, indented by a sinus which has a depth equal to 

 the width of the ears. The anterior ear of the left valve is one-half the 

 width of the posterior, but of nearly equal length; in the right valve the 

 ear is larger, and has a well-marked byssal sinus. 



The right valve is somewhat less convex than the left, but, with the 

 exception of the deeper anterior sinus, is very similar in character. 



Test ornamented by about sixty strong, rounded rays, with some smaller 

 intermediate elevated lines, which are crossed by regular, prominent, sub- 

 imbricating lainelhe of growth, about 1.5 mm. apart. On crossing the rays. 

 the varices of growth are bent suddenly upwards and backwards, forming 

 short semi-tubular spines. The strong rays are absent upon the ears, but 

 the lines of growth are numerous, and sharper than over the body of the 

 shell. 



The specimens are mostly partial casts of the interior, preserving no traces 

 of the muscular impression, pallial line, or hinge characters, but retaining 

 strong evidences of the external ornamentation of the shell. See fig. 7, pi. :i. 

 Numerous individuals have a height of 55 mm., with usually a Blightly 

 greater length. The length of the hinge-line in mature examples is about 

 45 mm. 



