HAMILTON GROUP. 153 



No. 1. Constricted orthocera (O. constrictum), which is very pecuhar, from the part narrow- 

 ing towards the mouth, and then expanding as if a Hgature had there been apphed. 



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. Chreat 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 whole 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 of 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 



