CLINTON GHODP. 



79 



469. 40. ATRYPA GIBBOSA («..9/;.). 



Pi.. XX. Fig. 10 a, b, c, d. 



Shell subglobose ; valves almost equally convex ; muscular impression, as marked in the 

 cast, shallow ; cast of dorsal valve showing the foliated muscular impression, and thin ab- 

 ruptly acute beak ; cast of ventral valve with a slit extending from the beak one third the 

 length of the shell, and the surface marked by faint longitudinal elevations and depressions. 



The casts of this species are not very milike those of A. reticularis., but the muscular im- 

 pression is less strong, and the depression on each side of the beak more marked, leaving the 

 latter more prominent, and the general outline of the cast is more rounded. 



Position and locality. In a hard siliceous layer in the central part of the group, near Clinton 

 village, Oneida county. (State Colhction.) 



470. 1. PENTAMERUS OBLONGUS. 



PL. XXV. Fig. la-m; and Pl. XXVI. Fig. 1 a, b, c, d. 



Pentamerus oblongus. Murchison, Sil. System, 1839, pag. 641, pl. 19, fig. 10. 



P. lavis. Ibid. Idem, pl. 19, dg. 9. 



P. oblongus. Hall, Geol. Rep. 4th Dist. N. York, 1843, pag. 7, fig. 1-5. 



Shell oblong oval or ovate, sometimes oblate or suborbicular, often depressed, becoming 

 ovoid in advanced stages of growth, also becoming sub-trilobate ; young shells smooth, 

 or marked only by fine scarcely elevated concentric lines ; older shells more or less strongly 

 marked by concentric ridges indicating stages of growth ; younger shells often exhibiting 

 two or three obscure longitudinal plications on each valve, in addition to the trilobate 

 character ; beak of the dorsal valve much extended, and closely incurved over the ventral 

 valve ; slope from the beaks more or less abrupt ; base rounded or subtrilobate. Cast of the 

 dorsal valve with the division made by the central longitudinal plate reaching about one third 

 the length of the shell ; cast of the ventral valve with the central chamber narrow and almost 

 linear, having a groove along the centre which reaches below the base of the separating plates, 

 the latter extending a little more than one third the length of the shell. 



It is impossible to give a description which shall be applicable to every form of a shell so 

 variable as this species. In the smaller and medium sized forms, in the Pentamerus limestone 

 of the Clinton group at Rochester and elsewhere, the shell has a general oval or ovate form, 

 sometimes slightly trilobate at base ; it is so much depressed that the thickness or depth of 

 both valves is only about half the width. This proportion sometimes continues even in very 

 old shells, the trilobate character of the base being often very conspicuous. In the majority of 

 specimens, however, the valves become gradually more gibbous as the shell increases in size, 

 and the trilobate character may be either preserved or entirely lost. In young shells, usually, 

 the surface is marked only by concentric stria? ; though in some instances distinct lines of 



