CHEMUNG GROUP. 
181 
Wood-cut 48 exhibits some of the fossils which belong to this group, and by which it may 
be distinguished from the others. 
No. 1. The Tricostate avicula (A. tricostata), so named from its showing three distinct sizes of ribs. Its 
surface and form make it a handsome shell. There are several aviculse in this group that have a 
general family resemblance; the most common and characteristic is the A. pecteniformis, which 
will be figured in Mr. Hall’s report of this group. It is believed to be the same with one in the 
Devonian system, from which it received its name. The tricostata belongs also to the same family. 
No. 2. Chemung cypricardite. This fossil was met with in several localities; the greatest number were 
at the small bridge on the road to Lisle from Binghamton. 
No. 3. Prolate delthyris (D. prolata). This is a very common shell of the group, and might at first 
sight be taken for the Mucronated strophomena of the Hamilton group; but the mesial part of the 
fossil is covered with fine ridges, and so also is the whole of the shell when perfect. It has also 
deep undulations upon the sides towards the extremities. The specimen was chosen from the State 
Collection, in consequence of showing a portion of the shell, and from the form of the internal part, 
and because it shows the structure of the hinge in part also. 
No. 4. Membranaceous strophomena (S. membranacea). This is an exceedingly abundant fossil; few 
probably more so of those belonging to the group. It is believed to be the same with one which 
is figured by Mr. Phillips in the Devonian system, and is named in accordance with that belief. It 
is a spinous shell, the spines often visible in the matter of the rock. There are but two shells 
of the kind in the New-York system; the other is in the Hamilton group, and the differences 
between them are not ascertained. 
No. 5. Supposed to be the upper valve of No. 4. 
No. 6. ■ delthyris, not named, but very abundant. 
