62 [Assembly 



differs from that one in the absence of the obscure crennlations or inequali- 

 ties upon the limb of the pygidium, which is regarded by Pictet as im- 

 portant. The number of segments of the thorax, if a constant character, 

 seems much more important, and furnishes a more marked feature for the 

 separation from Olenus. 



Geological position. In the shales of the Hudson-river group. 



Note. In addition to the evidence heretofore possessed regarding the position of the 

 shales containing the Trilobites, I have the testimony of Sir W.E.Logan that the 

 shales of this locality are in the upper part of the Hudson-river group, or forming a 

 part of a series of strata which he is inclined to rank as a distinct group above the 

 Hudson-river proper. It would be quite superfluous for me to add one word in support 

 of the opinion of the most able stratigraphical geologist of the American continent. 



Ftg. 3. Peltura holopyga. 



has apparently been refigured from the same specimen, or from the same figure through- 

 out, by subsequent authors; and the original appears to have been deprived of the 

 cheeks, the frontal limb, and the posterior cephalic spines. The eye-tubercle, or the 

 palpebral lobe, having collapsed as in our specimen, gives but a partial representation 

 of the entire animal. 



