LAMELLJBRANCHIATA. 39 



A.VICULOPECTEK TENUIS. 



PLATE VII, FIGS 27, 28 ; AMD PLATE LXXXI, FIG B 



Aviculopecten tenuis, Bali. Pal. N. V., vol. v, pt. I. Plates and Explanati 9: PL 7. Qgs. '-'7, 28. 



Jan., L883. 



Shell small to medium Bize, Bub-orbicular, transverse axis oblique ; Length and 

 height equal ; mar-ins regularly rounded, somewhat extended behind. 

 Lett valve slightly convex, nearly Hat. Right valve unknown. 

 HiiiiiX'-liiH- straight, nearly central,; length five-sixths of the Longitudinal 



diameter. 



Beak directed forward, not rising above nor extending beyond the hinge, 

 anterior to the centre of the hinge-line and of the valve. I'mhonal angle 90 c . 



Ears large, broad-triangular, unequal, separated from the sides of the umbo 

 by a broad, shallow sulcus. Anterior ear about One-half the size of the 

 posterior, tlat : margin convex; extremity obtuse-angular. Byssal sinus broad 

 and comparatively shallow. Posterior ear marked by a low, convex undula- 

 tion, adjacent ami parallel to the sulcus; margin straight or slightly concave: 

 extremity rectangular, pointed, more obtuse In mature specimens. 



Test marked by about twenty-live strong, regular radii, with broader 

 concave interspaces, and also, in well-preserved specimens, smaller interme- 

 diate rays. The specimens described are casts of the interior, and show hut 

 faint traces of concentric lines of growth. The ears are mostly free from 

 surface markings. 



Pallia] line impressed, continuous, situated about one-half the distance 



between the apex of the beak and the pallia] margin of the shell. Muscular 

 impression elliptical, within the concavity of the valves adjacent to the 

 sulcus of the posterior ear. Hinge-ligamenl without marked characters. 



Tin- Largest specimen has a height of 29 mm., length 30 mm., ami hinge- 

 line about 23 nun. A smaller individual is li.'! mm. in height, 23.5 mm. in 

 longitudinal diameter, and hinee-line measuring 19 nun. 



This species resembles in some respects .1. ignotus, and i> distinguished by 

 its transverse form, greater obliquity, Larger anterior ear in the left valve, 



