MEDINA SANDSTONE. 3 



Position and locality. This species is very abundant at Medina, Orleans county. It is found 

 at Lockport, a little to the north of the village ; and again two miles farther to the north, in 

 the extensive sandstone quarries of that town* I have found imperfect specimens at Rochester, 

 and tliin laminae in the sandstone are often made up of the comminuted fragments of these 

 shells. It is likewise of rare occurrence on the Niagara river, and at other localities along the 

 South shore of Lake Ontario. (*"«'« Collection.) 



This shell, together with the Jirthrophycus harlani, are unerring characteristics of the Medina 

 Sandstone. 



389. 22. ATRYFA OBLATA {n. sp.) 



Pl. IV. Figs. 4 a, b, c, and 5. 



Shell oblate, nearly as broad as high, broader at the upper part and sloping abruptly to the 

 beak, which is small, neat, and well defined ; valves nearly equally convex, the larger one 

 with a distinct groove down the centre ; the smaller valve presents a corresponding elevation, 

 with a slight groove on each side ; surface marked by fine concentric lines. 



This species is of a type distinct from any in the Lower Silurian, and one eminently cha- 

 racteristic of the Middle and Upper Silurian period : it is, therefore, of much interest, as 

 occurrincr at the very base of the upper division. There are several similar species in the Clinton 

 and Niagara groups, and in the Delthyris shaly limestone ; but this species is unknown in 

 either of these positions, though in form closely allied to some of them. Its distinguishing 

 feature is in the narrow longitudinal groove in the larger valve, which reaches nearly to the 

 beak, and is of nearly equal width throughout ; the corresponding elevation being narrow, and 

 unlike the broad mesial elevations and depressions of many species of this genus in a higher 

 position. (The specimen figured is a cast.) 



Fig. 4 a. View of the Ventral valve; 

 Fig. 4 b. View of base; 

 Fig. 4 c. Profile view. 



Fig. 5. The dorsal or larger valve imbedded in sandstone, which covers the lower part of the 



shell. It is impossible to say positively that this is identical with the preceding ; 



* but the partially imbedded specimen affords no positive characters to distinguish it. 



Position and locality. This species occurs in the upper gray portion of the formation at 

 Lockport ; and I am indebted to Col. Jewett, of that place, for the specimen figured. 



[Paleontology — Vol. ii.] 



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