OLENELLUS AND OTHER GENERA OF MESONACIDyE 343 



unlike that of OleiicHus canadensis [pi. 38, fig. 6], except that it is 

 close to the glabella at its posterior end. The glabella is comparable 

 with that of the very young specimens of Wanneria gracile [pi. 

 38, fig. 22], but in the adult forms of the latter the glabella is 

 broader at the occipital ring. The most nearly related cephalon is 

 that of O. fremonti, as expressed in some of its phases of growth 

 [pi. 27> figs. 14-16]. In these specimens the glabella is unusually 

 narrow and the eyes small and near the glabella, and the genal 

 spine is thickened more than usual, but, with the more expanded 

 anterior lobe of the glabella, outward inclination of the eyes, and 

 the impression obtained of the general assemblage of all the char- 

 acters of the cephalon as seen at one view, there is no danger of 

 confusing the two species. If we consider all the phases of the 

 cephalon of O. fremonti as shown on pi. 37 the two forms are at 

 once seen to be widely separated. 



We know nothing of the thorax and pygidium, but with such a 

 cephalon it is highly probable that strongly marked characters exist. 



PEACHELLA IDDINGSI (Walcott) 

 Plate 40, Figs. 17-19 



Olenellus iddingsi Walcott, 1884, Monogr. U. S. Geol. Survey, Vol. 8, 

 p. 28, pi. 9, fig. 12. (Described as a new species. The specimen repre- 

 sented by figure 12 is redrawn in this paper, pi. 40, fig. 17, other speci- 

 mens being used to restore broken portions.) 



Olenellus iddingsi Walcott, 1886, Bull. U. S. Geol. Survey, No. 30, p. 170, 

 pi. 19, fig. I. (Reproduces the description and figure given by Walcott 

 in 1884, and adds a paragraph on some specimens from a new locality.) 



Olenellus iddingsi Walcott, Holm, 1887, Geol. Foren. i Stockholm For- 

 handl., Bd. 9, Hafte 7, p. 515. (Described in Swedish.) 



Olenellus iddingsi Walcott, 1891, Tenth Ann. Rept. U. S. Geol. Survey, 

 p. 636, pi. 84, fig. 2. (No text reference. Figure 2 is copied from 

 Walcott, 1884, pi. 9, fig. 12.) 



Outline of cephalon roughly subtriangular with the length one- 

 half the breadth at the genal angles ; strongly convex in front and 

 sloping to the posterior margin, or the outer margin slopes up 

 toward the genal angles ; marginal border narrow and wire-like in 

 front and along the antero-lateral curvature of the border ; when 

 opposite the eyes the rounded border thickens and broadens so that 

 it passes into the genal spine with a size and convexity that gives 

 it the appearance of a distinctly elongated lobe; the genal spine 

 is short, and in the larger specimens almost blunt in its outline ; the 

 posterior border is faintly defined between the glabella and genal 



