HAMILTON GROUP. 
153 
No. 1. Constricted orthocera (0. constrictum), which is very peculiar, from the part narrow¬ 
ing towards the mouth, and then expanding as if a ligature had there been applied. 
No. 2. Recurved cypricardite (C. recurva), is a very characteristic, as well as a singular 
shaped fossil; like numbers 1 and 4, it is found only in this group. 
No. 3. Flabella avicula (A. flabella). It is an exceeding numerous species in the group, 
and is confined to it. It is readily known by its well defined form, its seven or eight 
large ribs, and its fine or small intermediate ones. The genus Avicula abounds in this 
group; no less than fifteen species being already named, and several others in the State 
Collection yet unnamed. Among the more common ones, is the Parity avicula (A. 
parilis), formerly the Monotis princeps. The Erect avicula (A. erecta) is also a common 
one, and is confined to this group. 
No. 4. Great orbicula (0. grandis). The under part only of the upper valve is figured. 
Near the centre, there is an indentation which resembles the navel of the human sub¬ 
ject, and from which a slight groove passes towards the circumference of the shell or 
disk. This fossil is readily recognized by the nearly flat circular form of the upper 
valve, and by the cap-like form of the lower one. 
The fossils figured in the two wood-cuts will serve to give some idea of those which are 
contained in this group ; others will be noticed at some of its prominent localities ; and a list 
of all those which have been named to this time by Mr. Conrad, will be given at the end of 
the group. There is no one of the groups which contains so many fossils that are restricted 
to it. 
The Hamilton group is confined entirely to the counties of Otsego, Madison, Onondaga, 
Cayuga, Herkimer, Oneida, Cortland, Chenango, and Tompkins. In the four first counties, 
it covers considerable surface; in the last five, but very little. 
In Otsego county, the group covers the hills on both sides of the valley at Cherry-Valley, 
extending thence over the whole of the town to the south, forming the surface rock of Decatur 
and Westford. It covers the wdiole of Middlefield, and its north extension, not named upon 
the maps of the State; also the upper part of the town of Milford; all but the southwest 
corner of Hartwick ; the whole of the town of Otsego, extending up to the head of Schyller’s 
and Otsego lakes. It covers the whole of Burlington, Edmeston, Plainfield and Exeter ; the 
lower part of Richfield, and the northern part of Pittsfield and New-Lisbon. 
In Madison county, it forms one half of its area, covering the whole of the town of Brook¬ 
field, excepting a triangular projection towards the southwest end. It covers the whole of the 
towns of Hamilton and Madison, the larger east half of Lebanon, the greater part ot Eaton, 
the west-southwest portion of which is covered by higher rocks. It covers the extreme south 
part of New-Stockbridge, the lower half of Smithfield, the line passing from southeast to north¬ 
west; all of Fenner, excepting the north and the northwest part. It covers the largest portion of 
the town of Cazenovia, extending on the west side from its south line, to about the head of 
the lake. The lower part of the town where it joins on Nelson, contains higher rocks ; the 
dividing line between them and the Ithaca group passing by the northwest corner of Nelson, 
Geol. 3d Dist. 20 
