CHAZY LIMESTONE. 19 



SHELLS, TRILOBITES, <$-c. OF THE CHAZY LIMESTONE. 



Plate IV. (bis). 



28. 1. LEPT^NA PLICIFERA («. i7).), 



Pl. IV. (bis). Figs. 1 a, b. 



Senii-cUiptical, much wider than long ; convex valve slightly depressed along the centre, 

 from beak to base ; surface marked by about 34 or 36 equal, somewhat coarse stria>, which 

 radiate, apparently in pairs, from the apex of the shell, intermediate ones often appearing 

 towards the margin ; stri;E sharp, with rounded grooves between ; visceral cavity, as shown 

 by the cast, duplicate, separated by a sharp ridge, which extends towards the base of the 

 shell. 



Width 1^ to § of an inch ; length j\ to \ of an inch. 



The condition of nearly all the shells of this species, which I have seen, is such as to 

 render clear and definite descriptions almost impossible. It is mostly preserved in impres- 

 sions of the outer shell, or as casts. It is quite unlike any species in the Trenton limestone, 

 but approaches in some degree a small one in the Hudson-river shales. 

 Fig. 1 a. Figure of the convex valve. 

 Fig. I b. Cast of convex valve, witli a portion of the shell remaining : cast of visceral cavities magnified. 



Position and locality. This occurs in great numbers in some thin layers of limestone near 

 the higher part of the mass, at Chazy, Clinton county. 



29. 2. LEPT^NA INCRASSATA ( n. xp.). 



Pl. IV. (bis). Figs. 2 a, b, c. 



Shell thickened, almost semicircular ; dorsal valve very convex near the beak, gradually 

 becoming flattened on the disc, and suddenly bending upwards near the margin ; ventral 

 valve moderately concave, exhibiting a tliiekening near the margin ; surface marked by 

 sharp elevated radiating lines, between which are broader spaces. 



The surface of this species presents a kind of thickening which partially obliterates the 

 striae ; the ventral valve particularly exhibits this sort of incrassation. It occurs with the 

 last described in considerable ntimbers, but differs in being larger, thicker and more convex, 

 as well as in the form of the visceral markings. This species often bears some resemblance 

 to L. interstrialis of the Trenton limestone, but is a much smaller shell, and differs from 

 the young of that species in essential points. 



Fig. 2 a, b. Convex valves of this species. 



Fig. 2 c. Ventral valve. 



Fig. 2 d. Cast of the interior of the convex valve, and the same enlarged. 



Position a)id locality. In the thiiil)ed(led limestone, upper part of the mass, Chazy, 



Clinton county. ^stale Collection.) " 



3" 



