236 (Sept. 21, 
Claypole.] 
The Hamilton sandstone is a peculiar deposit of sand in the midst of a 
vast accumulation of shales. It covers a district extending from the Blue 
mountain northward for about fifty miles ‘and eastward to the nvigh- 
borhood of the Schuylkill river. Westward its limit cannot be traced, as 
it is destroyed by erosion, but from appearances it was as great as in the 
vast. It lies between a mass of shale above and another mass below, and 
at its greatest development is about 800 feet thick, at the Susquehanna 
gap. Some of its beds, especially toward the middle, are very hard and 
flinty, but it grows more and more shaly as it recedes from this point. 
Apparently it exists at some distance from its point of greatest develop- 
ment as a sandstone mass below and another above, with intervening 
shales. 
Note on a large Crustacean from the Catskill Group of Pennsyluania. By 
H. W. Olaypotle. 
(Read before the American Philosophical Society, Sept. 21, 1883.) 
I have lately received from Mr. R. D. Lacoe, of Pittston, a slab of green 
sandstone, from the Catskill group of Wyoming county, containing a well- 
preserved head of some creature. Though not complete, yet enough re- 
mains to enable me to form a good idea of what the full form of the head 
must have been. 
It measures eight and a half inches across the broadest part, and the 
same from front to back. The outline is semi-elliptical, the part preserved 
corresponding to a piece cut from one of the ends of an ellipse. It is 
somewhat distorted, and may when perfect have been more nearly semi- 
circular. The outline is slightly wavy, but this also may be due to distor- 
tion. Fortunately the right side is almost perfect and, being symmetrical, 
it is not difficult to reconstruct the other. A good idea of its general shape 
may be suggested to a paleontologist by saying that it resembles the head 
of Cephalaspis. 
A longitudinal median ridge runs from near the front margin almost to 
the back of the portion preserved, dividing the head surface into two 
equal parts. This ridge rose near its front end into a low tubercle, or 
perhaps a spine, and near its hinder end into a distinct and boldly elevated 
spine which is, however, crushed down almost flat. Posteriorly the ridge 
narrows and tapers down to the general surface. 
At the place of the posterior spine another ridge, less distinct, crosses 
the former at right-angles, and itself rises at its two ends, midway to the 
outer margin, into low prominences from which two semicircular ridges, 
convex outwardly, run curving in toward the median line at both their 
ends, one in front, the other behind the cross-ridge from which they start. 
Each cross-ridge, with its semicircular branch, resembles in outline an 
