10 PALJEONTOLOGY OF NEW YORK. 



Ilin^ro-liiie straifrlit, t'xteiidinj: a.s lar as tliu aiitL-riur inargiu ; k'ligth two- 

 thinls thf lioijrlit of tlif sliell ; li<:;ainental area narrow. 



Boaks well-delined, uot rising aljove the hinge-line, situated anterior to 

 the middle. 



Posterior ear large, triangular, margin concave ; the ornamentation differ- 

 ing from that on tlie iunl)o ; defined hy a sliirlit depression and a rliange of 

 convexity. Anterior ear small, defineil hy a wcU-niiirkcd sulcus; hyssal 

 sinus deep; margin convex near the extremity of the iiinge-line. The area 

 of the posterior ear is more than twice that of the anterior. 



Test ornamented hy ahout 35 rounded, radiating costa% with intermediate 

 smaller ones, crossed and crenulated hy regular striie of growth, of which 

 there are ahout 10 in the space of 5 mm. On the ears the radiating lines 

 are present, especially on the anterior ear, and the concentric striae are more 

 crowded than on the Ixxly of the shell. 



The characters of the interior are not preserved. 



The largest specimen has a length of 32 nun., and tlie liinuc-line measures 

 23 mm. A smaller specimen, preserving Imth valves, has a length of 'JO 

 mm., and height ahout equal. 



This species is distinguished from A. exacutus hy the greater angle subtended 

 1(\ the sides of the mnho. ])\ its greater obli(piity, and the less sharp radiating 

 costjB of the surface. In A. Phorcus the length is comparatively greater and 

 hinge-line .shorter, the ears smaller, and the strong crenulatiug stria) of 

 growth do not occur. 



Formation and localities. In the shales of the Hamilton grouj), at Fultonham, 

 iSchoharie (■oiuity ; and Hainilton, Madison county, N. Y. 



Avit'ULurECTEN Phorcus. 



IM.ATK V, KlU. •►;. 

 Atyicttloperten Phorcwi, Ham,. Piil. N. V., vol. v. jit. 1. Plates and Explanations: PI. 5, ti),'. 22. .Jan., l.'<83. 



Shem. of medium size, ohliquely ovate ; height and length equal ; anterior and 

 ba.sal margins regularly rounded, becoming more convex towards the middle 



