LAMELLIBRANCHIATA. 16 



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

 defined by the umbonal carina 1 , 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 car 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 leffl valve. 



Test thin, ornamented by about '.•<) sharp rays, with concave interspaces 

 having an equal number of intermediate liner rays, and crossed l>\ numerous 

 regular, sharp concentric stria'. The markings on the ears are similar to 

 those on the body, hut less strong. 



Ligamental area very narrow, and marked on the outer margin by an 

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



The left valve of one specimen has ;i height of 23 mm.. Length "J 1 mm.. 

 hinge-line 23 mm.; the right valve of the same has a height of 22 mm., and 

 a length of '2A 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 .1. 

 scabridus may lie 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, X. V. 



A\ [CULOPECTEH lin..KSTRIATUS. 



PLATE Ml. KIG3. 8 11. LXD PLATE 1.XW1. FIGS II. IS. 



Limn rtitjastniita. Hall. Geolog. Surv. N. 'i . : Rep. K tli Mat., p. '1*'A. 1843 



AvictUopectc* riK/n.si, i.itii.*. Hall. Fill. N. V.. rol. v, pt. 1. Plates and Explanations: PI. 7. figs. 8 II. 



Jan . 1883. 



Shell longitudinally ovate, oblique, more convex in front ; height one-fourth 

 greater than the length; outline oblique-ovate, with the posterior margin 



