LAMELLIBRANCHIA TA. 1 5 



concave except in the anterior ear of the right valve, which is convex ; 

 defined by the umbonal carinte, 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 umljo. 



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 stria3. The markings on the ears are similar to 

 those on the l)ody, but less strong. 



Ligamental area very narrow, and marked on the outer margin ])y an 

 inflection of the test. The other characters of the interior are not preserved. 



The left valve of one specimen has a height of 2.3 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, l)ut 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 

 difiers by its oliliquity 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 RUG^STRIATUS. 



PLATE VII, FIGS. 8-11 ; and PLATE LXXXI, FIGS. 11, 12. 



Lima rugwstr'iata, IIali.. Geolog'. Surv. N. Y. : Rep. Fourth Dist., ji. 264. 1843. 



Avictilupecteii rugiextiiatiis, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 7, fig's. 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 



