712 THE PALEONTOLOGY OF MINNESOTA. 



[Nlleus vigilans. 



grooves which are linear. These sulci disappear at reaching the broad margin, but 

 the interannular grooves are continued upon, and nearly across the marginal 

 expansion. The surface appears to have been quite smooth. 



The larger of the specimens measures 2 mm. both in length and width. 



The distinctive features of this subgenus and species are those of early 

 representatives of the asaphid type, which at maturity show a condition of distinct 

 annulation. The form appeared at a period when the true asaphs were on the 

 decline and near extinction. Both structure and size indicate that this was a 

 paracmic modification of the asaphid stock, reproducing in the senility of the race 

 the characters of immaturity. 



Formation and locality. Utica shale, mouth of Licking river, Ohio; probably also in the Hudson 

 River shales, Rome, N. Y. 



Genus NILEUS, Dalman, 1826. 

 NILEUS VIGILANS Meek and Worthen, (sp.) 1875, 



Asaphus vigilans MEEK and WORTHKN, 1875. Geol. Surv. Illinois, vol. vi, p. 497, pi. 23, flg. 6. 

 lllirnitx (Nileus) minnesotensis, FOEKSTE, 1887. Fifteenth llept. Geol. and Nat. Hist. Surv. Minn., 

 p.,478, fig. 1. 



The description given by Mr. A. F. Foerste was based upon a single cranidium 

 from the Trenton horizon at Minneapolis. There are before me a number of 

 essentially entire individuals, most of them enrolled but several in an extended 

 condition. Some of these are from Minneapolis but the majority from the Galena 

 beds. Upon comparison of these with the description and original specimens of 

 Asaphus vigilms Meek and Worthen, I find no basis of specific distinction. 



Fig. 17. Nilerts vigilans Meek and Wortln'ii. Fi ^ ls . -i' r ,,|]] ( . ,,f u, ( . Sillll( . 



Cephalic view of an etindlc-d individual. 



General form elliptical with subcrescentic extremities; longitudinal lobation 

 very obscure. Cephalon transverse, regularly convex. Margin very slightly 

 thickened by a thread-like elevation. Genal angles obtuse. Frontal slope full but 

 not projecting, terminating abruptly on the margin; lateral surface slightly 

 depressed beneath the eyes; upper surface between the eyes flattened. Glabella, 

 dorsal furrows, occipital furrow and ring not defined. Eyes small for this genus, 

 but prominent; situated at points each one-third of the entire transverse diameter 

 from the lateral margin and one-third of the longitudinal diameter from the posterior 



