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 O. carinata is also fotmd in this rock, but it is 

 rare. 



No. 3. Mucronated or Sharp-pointed delthyris. This truly graceful shell exists in great 

 nimibers 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 specits 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 ortbocen (0. coimtrictuin). 

 2. Rcenrred C7pricirdite« (C. recarrt). 



No. 3. Flabella aricula (A. flabella). 

 4. Great orbicula (O. grandia). 



