tt] PALEONTOLOGY — UPPER MISSISSIPPI. 225 



and the genus Obolella of Billings is described with special reference 

 to the species Obolella polita (pp. 131-134). 

 The new genera described are: 



Lingulepis, p. 129. 

 Ptychaspis, p. 170. 

 Charipcephalus, p. 175. 

 Illaenurus, p. 176. 

 Triarthrelia (subgenus), p. 177. 

 Aglaspis, p. 181. 

 Peiiipbigaspis, p. 221. 



The genus Conaspis is proposed in the event of a new generic group 

 being formed of Conocephalites perseus, C. shumardi, C. nasutus, C. 

 oweni, C. eryon, C. anatinus, and C. pater soni (p. 152). 



.Prof. Hall states that he was unable to recognize the six successive 

 trilobite beds of the saudstoue as indicated by Dr. Owen, but he 

 nevertheless refers the species described to three different epochs of 

 the Potsdam period, as follows : 



In the lower bods of the formation I have fouud Conocephalites proper, together 

 with Lingula, Lingulepis, Obolella!? and Theca. In the middle stage, neither the 

 limitsof the beds, nor the range of species or genera, have been so well determined; 

 but, grouping together all that I have found between the well-defined upper beds 

 and the lower fossilferous beds known, we have Conocephalites, Dikelocephalus, Ari- 

 ouellus, Ptychaspis, Chariocephalus, Ilhenurus and Aguostus, in the trilobitic fauna, 

 together with Orthis and Platyceras. 



The Graptolitidto apparently begin their existence somewhere in this central epoch, 

 but their precise relations to the other beds have not been determined. 



In the higher beds of the formation, and clearly separated from the great central 

 mass, wo have the genera Dikelocephalus, Triarthrelia, and Aglaspis, together with 

 Lingula, Serpnlites, and Euomphalus. 



We observe, therefore, that the earliest trilobites are referable to the genus Cono- 

 cephalites; and the genus Dikelocephalus does not appear in the first stages of the 

 formation, nor below the beds which I have referred to the second or middle stage of 

 the period. There this genus appears in three species, smaller and less conspicuous 

 than those in the higher beds. It is only in the later stages of the sandstone that 

 the typical species of this genus of Dr. Oweu appear, and those from the lower beds, 

 thus referred by him, belong apparently to other genera. 1 



From a small collection of fossils from the Potsdam sandstone of 

 Sauk County, Wisconsin, Dr. Alexauder Wiuchell describes in 1804 

 four new species and mentions the presence of some others that have 

 been described. 2 The new species are : 



Orthis barabuensis, p. 228. 

 Straparollus (Ophileta) primordialis, p. 228. 

 Pleurotomaria ? advena, p. 228. 

 Ptychaspis barabuensis, p. 230. 



He mentions the occurrence of Scolithus linearis (p. 227) and Dikel- 

 ocephalus minnesotensis and Dikelocephalus pepinensis (p. 229). From 

 the Lake Superior sandstone he describes Palceophycus articulatus (p. 

 231) and P. informis (p. 232). The two latter species are doubtfully re- 



•Op. cit.,p. 183. 



'Notice of a small collection of fossils from the Potsdam Sandstone of Wisconsin and Lake Superior 

 Sandstone of Michigan. Am. Jour. Sci., 2d aoi\, vol. 37, 1864, pp. 226-232. 



Bull. 81 15 



