LAMELLIBRANCHIA TA. 
15 
concave except in the anterior ear of the right valve, which is convex; 
defined by the nmbonal carinae, and by the absence of rays on the posterior 
slope. Byssal notch in the right valve angular, deep, continuing to the 
beak as a sulcus which separates the ear from the umbo. 
The right valve, though very similar to the left, is not so high, and the 
anterior ear differs in form from the corresponding one in the left valve. 
Test thin, ornamented by about 90 sharp rays, with concave interspaces 
having an equal number of intermediate finer rays', and crossed by numerous 
regular, sharp concentric striae. The markings on the ears are similar to 
those on the body, but less strong. 
Ligamental area very narrow, and marked on the outer margin by an 
inflection of the test. The other characters of the interior are not preserved. 
The left valve of one specimen has a height of 23 mm., length 24 mm., 
hinge-line 23 mm.; the right valve of the same has a height of 22 mm., and 
a length of 24 mm. A larger specimen is 29 mm. in height, with an equal 
length. 
This species resembles A. exacutus in outline, but differs in the lesser con¬ 
vexity and obliquity of the valves; the beaks are not prominent, and the 
surface characters are finer and more delicate. Young individuals of A. 
scabridus may be distinguished from this species by the more rounded and 
obtuse beaks, and rugose rays, which are usually absent on the ears. A. formio 
differs by its obliquity and the inequality of the ears, and the much stronger 
striae. 
Formation and localities. In the shales of the Hamilton group, at West 
Bloomfield, Ontario county, and York, Livingston county, N. Y. 
AviCULOPECTEN RUGiESTRIATUS. 
PLATE VII, FIGS. 8-11; AND PLATE LXXXI, FIGS. II, 12. 
Lima rugaestriata, Hall. Geolog. Surv. N. Y.: Rep. Fourth Dist., p. 264. 1843. 
Avieulopecten rugasxtriatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PL 7, figs. 8-11. 
Jan , 1883. 
Shell longitudinally ovate, oblique, more convex in front; height one-fourth 
greater than the length; outline oblique-ovate, with the posterior margin 
