138 S. W. Ford—Distribution of the Fossils in the 
to the pygidium, at least it invariably accompanies it when the 
latter is found isolated. 
more rapidly tapering; gracefully curved in outline. Marginal 
rim distinct all around, widest anteriorly, distinctly raised or 
thickened at the edge. xis conical, sometimes acutely so, 
long and slender, extending very nearly to the margin, divided 
by faint cross furrows directed slightly backward into eleven 
rings or segments. Side-lobes highly convex and without fur- 
rows. The axis and side-lobes appear to overhang the marginal 
rim at the posterior extremity, giving the border the appearance 
of being only about half as wide behind as it is in front. 
The entire surface is finely punctate. In one specimen 
of the pygidium, out ofa large number of perfect specimens 
examined, there appears to be a twelfth ring in the axis. 
Length of a specimen of the usual size with all the parts in 
place, but too imperfectly preserved to be figured, half an inch. 
Length of thorax 0°18 of an inch. I have seen but a single 
specimen showing conclusively the true number of thoracic seg 
ments. This species had the habit of rolling itself up into a 
ball, and is quite often found in this state. In the specimen 
just noticed, the pygidium is bent slightly under the body. 
Nearly all of the specimens that I have seen, that were in a per- 
fect condition before extraction, exhibit this tendency to coil 
themselves up, which appears to account for the thorax break- 
Ing away 1n most cases. 
This pretty little trilobite occurs in both even-bedded and 
conglomerate limestones of the Lower Potsdam at Troy; also 
at Bic Harbor, Canada, where it has been collected by Mr. T- 
C. Weston of the Geological Survey of Canada. The Troy 
specimens were collected by the writer. It is a rather common 
fossil at Troy, the head and pygidium usually occurring de 
tached from the thorax. The head sometimes attains a length 
of 0°38 of an inch, but this is quite unusual. 
This species closely resembles Microdiscus punctatus (Salter), 
from the Menevian group of Wales;* but it differs from that 
species in not possessing a neck spine, in the greater number 0 
its caudal rings, and in having the marginal rim of the head 
tuberculated. The head, compared with that of Microdiscus 
Dawsoni from the St. John’s group of New Brunswick (Acadiat 
Geology, Dawson, 1868, p. 654), is proportionally longer, a0 
is wanting in the grooves of the border of that species. 
Leperditia Troyensis,+ sp. nov. 
The following description is based upon a single right valve, 
the only specimen of this species that I have seen. 
* Quart. Jour. Geol. Soc., Aug., 1864, p. 237. 
+ Figures of this and the following species will be given hereafter. 
