152 
GEOLOGY OF THE THIRD DISTRICT. 
Dipleura of the same size, the largest not over an inch in length. The only difference 
observed between them, was in the tail. This fact settles the question of difference as 
to genus, which with the British geologists appeared to be doubtful. The specimens are 
in the Collection of the State. With the exception of two instances only in an upper 
position, this trilobite is confined to the Hamilton group. It is rare in the fine slate or 
shale, but common in the coarse shale and sandstone. 
No. 2. Undulated orthonata. This very beautiful shell is confined altogether to this group. 
It is not an abundant one, but is found from one end of the district to the other, showing 
at least diffusion of its individuals. The 0. carinata is also found in this rock, but it is 
rare. 
No. 3. Mucronated or Sharp-pointed delthyris. This truly graceful shell exists in great 
numbers in the group, and rather generally diffused. It takes its name from its pointed 
extremities. It has the appearance at first sight of a butterfly, and is often taken for 
such. The form of this species is not always so much extended in the direction of the 
line of the hinge, as in the one figured; those of this kind probably having obtained to 
their perfect state as to form, whilst others are shorter, and of greater breadth from the 
hinge outwards, appearing at first view like distinct species, but no other difference exists 
except a few additional ribs where elongated. 
In the Report on the fourth district, there is a wood-cut figure of the Microdon bellastriata, 
and of the Bellerophon hamiltoni, both very characteristic fossils of the Hamilton group. 
The wood-cut No. 37 below presents also four common and exclusive ones, selected from 
numerous others which belong to this group. 
37. 
No. 1. Constricted orthocera (0. constrictum). No. 3. Flabella avicula (A. flabella). 
2. Recurved cypricardites (C. recurva). 4. Great orbicula (O. grandis). 
