OLENELLUS AND OTHER GENERA OF MESONACIDiE 317 



back of the eye lobe by a raised line that extends from the base of 

 the eye lobe backward and slightly outward to the posterior margin 

 of the head at the intergenal angle when the latter is present: this 

 corresponds in position to the facial suture in the genus Paradoxides. 



Numerous fragments of the thoracic segments have been found in 

 association with the cephalon, but nothing is known of the number 

 of segments or the character of their axial lobe. Two fragments 

 of the pleural portion of the segment are illustrated by figs. 9 and 10. 

 These indicate similar characters to those of the segments of Olenel- 

 his thompsoni (Hall) [pis. 34 and 35]. 



The telson [fig. 8] is known only by fragments. It is an elongate, 

 slender telson without segments or lateral lobes, in this respect re- 

 sembling the telson of Olenellus thompsoni Hall [pi. 35, fig. i] and 

 0. fremonti Walcott [pi. 37, fig. 7]. 



Hypostoma moderately convex, broad in front and narrow toward 

 the posterior margin. The anterior margin shows a rounded, smooth 

 edge that fitted into a curved recess in the doublure of the head. 

 The lateral margin forms an elevated rim for a short distance, and 

 then curves downward to the more elevated posterior rim ; the 

 posterior marginal rim is separated from the body by a sulcus that 

 disappears on each side ; a second groove or sulcus arches across 

 so as to represent a narrow lobe, as shown by fig. 3. A large num- 

 ber of more or less crushed specimens of the hypostoma were found 

 associated with the fragments of the cephalon and thorax. 



The surface of the head and the fragments of thoracic segments 

 have the characteristic Olenellus marking. It forms an inosculating, 

 fine, raised fretwork. This type of surface is beautifully shown by 

 figs. 4 and 5, pi. 37, of this paper. 



Dimensions. — The largest specimen of a cephalon has a length of 

 4.5 cm., and a width of 7 cm. A small head 4 mm. in length has a 

 width of 7 cm. 



Observations. — The presence of the genus Olenellus in the Rocky 

 Mountain regions of British Columbia has long been known. In 

 1886 I identified for Dr. Geo. M. Dawson, of the Canadian Geological 

 Survey, among the fragments of fossils found at Kicking Horse 

 Pass, a species of Olenellus that appeared to be Olenellus howelli 

 Meek.' During the summer of 1907 I visited the Kicking Horse 

 Pass and made an examination of the strata in which Olenellus 

 occurs. The preliminary study of the fragmentary material collected 



' This is the species that I placed with Olenellus gilberti [Walcott, 1886, pp. 

 164 and 170]. 



