OLENELLUS AND OTHER GENERA OF MESONACID.E 33] 



OLENELLUS GILBERTI, var. 



Plate -40, Fig. 8 



This small cephalon 3.5 mm. in length occurs in association with 

 Olcncllus canadensis [pi. 38] and O. o^ilberti [pi. 36]. Its large 

 eye and broad space between the marginal rim and glabella dis- 

 tinguish it at once from O. canadensis, and its stronger marginal 

 rim and very strong ridge connecting the anterior lobe of the glabella 

 with the elevated eye lobe distinguish it from cephalon s of the same 

 size referred to 0. gilherti. It is unlike the latter, but it has so 

 many points in common with it that I will designate it as 0. i^il- 

 berti var. 



Formation and Locality. — Lower Cambrian : Mount Whyte 

 formation: (35I) limestone layer above the arenaceous beds of the 

 St. Piran formation and below an arenaceous shale, on the south 

 slope of Ptarmigan Pass, near head of Corral Creek, 9 miles (14.4 

 km.) north-northeast of Laggan, Alberta, Canada. 



OLENELLUS LAPWORTHI Peach 



Plate 59, Figs. 1-7; and Plate 40, part of Fig. 3 



Olcncllus lapworthi Peach and Horne, 1892, Quart. Journ. Geol. Soc. 



London, Vol. 48, pp. 236-241, pi. 5, figs. i-ii. (Described and dis- 

 cussed.) 

 Olcncllus lapzi'orthi Peach and Horne, Peach. 1894, Quart. Journ. Geol. 



Soc. London, Vol. 50, pp. 662-664, pi. 29, figs, i, 2. and 2a ; pi. 30, fig. 7 ; 



pi. 32, fig. 8. (Described and discussed.) 

 Olenellus lapzcorthi clongatus Peach, 1894, Idem, p. 664, pi. 29, figs. yS. 



(Characterized as a new variety. The specimen represented by figure 



3, pi. 29, is redrawn in this paper, pi. 39, fig. i.) 

 Olenellus interniedius Peach, 1894, Quart. Journ. Geol. Soc. London, Vol. 



50, pp. 666-668, pi. 32, fig. 7. (Described and illustrated.) 



Olenellus lapworthi belongs to that group of the Mesonacidae rep- 

 resented by 0. thompsoni [pi. 35], O. gilherti [pi. 36], and 0. fre- 

 monti [pi. 37]. Its relation to the adult of 0. thompsoni may be 

 seen by comparing figs. 2-4, pi. 39, with fig. 9, pi. 34. A young 

 stage of O. lapworthi [pi. 39, fig. 8] may be compared with a young 

 specimen of Pcedeumias transitans [pi. 32]. Also compare the hyp- 

 ostoma [fig. 7, pi. 39] with that represented by fig. 7, pi. 34. 



Olenellus lapzvorthi differs from O. thompsoni: (a) in having a 

 shorter eye lobe that extends more obliquely outward; and (b) in 

 the geniculation of the pleurae of the thoracic segments which is 

 more abrupt [compare fig. 2, pi. 39, with fig. i, pi. 35]. The eye 



