10 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Its middle lobe is more prominent than the lateral, and shows three segments ; the 

 lateral lobes appear to have only two segments. 



Compared with Prof. Barrandc's beautiful figures of the variable typical species, 

 A. ceticephalus, of its own size, the head of our species is more regularly rounded 

 in front, and differs in having a distinct anterior marginal furrow, while its glabclla 

 is proportionally longer. A specimen of a much smaller individual in the same 

 matrix, shows this species to have been also much less variable in the characters 

 just mentioned, at different periods of its growth, than A. ceticephalus. Again, if 

 the pygidium mentioned above belongs to the species under consideration, it was 

 larger in proportion to the largest head we have seen, than in the European species. 



Dr. Shumard and Mr. Billings, to whom we sent sketches of our species, regard 

 it as being clearly distinct from any of the forms described by either of them from 

 the Primordial rocks of Texas and Canada. 



As the specimens of this Trilobite came in after our plates were made up, we 

 could not well introduce figures of it there, but the annexed wood-cuts will give a 

 tolerably good idea of its general appearance. 



Locality and position. Near the head of Powder River, in the Big Horn Moun- 

 tains, Dakota Territory. From the Primordial or Potsdam Sandstone Group. No. 

 1180, collection of the Smithsonian Institution. (Type 1180a.) Discovered by the 

 Exploring Expedition under the command of Captain William F. Raynolds, U. S. 

 Topographical Engineers. 



Agraiilos? 



(PLATE I, Fig. 4.) 

 Comp. Crepicephalux, OWEN, Report Geol. Wisconsin, Iowa, and Minnesota, 1852, PI. I. A. Fig. 18. 



The specimen of this species we have figured is an internal cast of a part of a 

 cephalic shield, in a coarse brown sandstone. It is not in a condition to show any 

 traces of the facial sutures, or even the form of the entire glabella, nor the position 

 of the eyes, if they exist. Hence we cannot determine with much confidence to 

 what genus it belongs, nor can we give any characters by which the species can be 

 identified. As near as can be determined, it seems to be similar to some of the 

 forms figured by Dr. Owen under the name Crepiceplialus. It also resembles the 

 foregoing species from Big Horn Mountain, and may possibly be the same. Still, 

 as it presents some slight differences, it may prove to belong to another species. 

 Although a mere fragment, we have thought it should be figured, since, as far as 

 its affinities can be made out, it corroborates the evidence of the other fossils in 

 regard to the age of the formation. 



Locality and position. Central part of the Black Hills, Dakota Territory. Pots- 

 dam or Primordial Sandstone. No. 1024. 



