MEDINA SANDSTONE. 
9 
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 thin lamime 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. {State Collection.) 
This shell, together with the Arthrophycus harlani , are unerring characteristics of the Medina 
sandstone. 
389. 22. ATRYPA OBLATA (». 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 
occurring 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 tb 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 depressitiiis 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 j 
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 ; ] 
