OLENELLUS AND OTHER GENERA OF MESONACIDyE 329 



a wide geographic distribution in the Cordilleran area of the United 

 States and Canada, and a possible stratigraphic range of several 

 hundred feet in the upper portion of the Lower Cambrian (Georgian) 

 formations. Its representative on the eastern side of the continent 

 in the Appalachian area is Olenellus thompsoni [pi. 34], and in 

 Scotland 0. lapzvorthi [pi. 39]. In Canada a form that I have identi- 

 fied with this species occurs in the Mount Whyte formation at several 

 localities, the most prolific of which is at Ptarmigan Pass, Alberta 

 (locality No. 35I), where a number of small and large cephalons 

 were found in a thin layer of limestone at the base of an argillaceous 

 shale. Fragments of 0. canadensis are also abundant in this lime- 

 stone and in the arenaceous beds beneath. Another notable locality 

 is near the base of the Mount Stephen section (locality No. 35f), 

 about 300 feet below the summit of the Lower Cambrian. 



Comparison with other species of Olenellus shows that O. gilberti 

 differs from 0. fremonti [pi. 37] : (a) in having its glabella sepa- 

 rated from the frontal border by a clear space ; (b) in having a 

 longer, larger palpebral lobe and eye ; (c) in having the third thoracic 

 segment very little, if any, larger than the fourth and fifth; (d) 

 in having the pleural lobes proportionally wider. The two species 

 are associated in eastern (locality no. 30) and western Nevada 

 (locality no. ip). 



From O. thompsoni it differs in the less expanded anterior lobe 

 of the glabella and the space in front of the lobe. 



From O. lapworthi [pi. 39, figs. 1-8] it differs in many minor de- 

 tails and most notably in the form of the thorax and thoracic 

 segments. 



The most nearly related cephalon is that of P^denmias transitans 

 [pi. 34], and the thorax of the two species is very similar back to 

 the large spine. If 0. gilberti should be found to have rudimentary 

 segments and pygidium posterior to its telson-like spine the two 

 forms would probably be placed under the species gilberti of the 

 genus Pcedenmias. 



Formation and Locality. — Lower Cambrian: (31a) in dark, 

 fine, arenaceous shales and interbedded thin layers of limestone in 

 the Pioche formation, on both the east and west slopes of the anti- 

 cline of quart zitic sandstone at the mining cam-p of Pioche; and 

 (30) west slope of Highland Range 8 miles (12.8 km.) north of 

 Bennetts Spring and about 8 miles (12.8 km.) west of Pioche; both 

 in Lincoln County, Nevada. 



