328 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Dimensions. — The largest cephalon has a length of 41 mm., width 

 60 mm., convexity 13 mm. [figs. 4 and 5]. The only specimen of the 

 thorax is too much distorted to base measurements upon it. 



Young stages of growth. — Some of the younger stages of growth 

 of the cephalon are beautifully preserved in a compact, dark lime- 

 stone from Ptarmigan Pass (locality No. 35I). A few are illus- 

 trated. These show that in the cephalons from 2 to 6 mm. in length 

 there is considerable variation in outline. The smallest have a sub- 

 quadrilateral outline with a distinct antero-lateral angle and short 

 spine [ligs. 11-13]. As the cephalon increases in size the angle 

 and spine disappear, and the evenly rounded outline is unbroken 

 from the genal angles to the broadly rounded front margin [figs. 

 15-17]. The palpebral lobes of the smallest cephalon, 1.75 mm. 

 in length, terminate posteriorly opposite the third glabellar furrow 

 [fig. 11], and this continues up to specimens 15 mm. in length 

 [fig. 15], but in the large cephalons 20 to 40 mm. in length the 

 palpebral lobe is proportionally larger and ends at the furrow within 

 the posterior margin of the cephalon [figs. 1-4, 16 and 17]. The 

 space between the frontal lobe of the glabella and the anterior wire- 

 like border of the cephalon varies slightly, but it is rarely that it is 

 narrower than the width of the frontal border. The intergenal 

 spines [figs. 11-14] are the continuation of the ridge running from 

 the base of the eye that appears to represent the line of the facial 

 suture back of the eye ; the antero-lateral spines arc in tlie position 

 where I should anticipate finding the termination of the facial suture, 

 in front of the eye. The comparison of the young stages of growth 

 of this species with similar known stages in other species is made 

 in the introduction [pp. 236-243]. 



An hypostoma occurring (localit}- No. ih) with specimens of the 

 cephalon of this species has a denticulated posterior margin much 

 like that of the hypostoma of Wanneria halli [pi. 31, fig. 9], and 

 Olenellus fremonti [pi. 37, figs. 21, 22]. It is strange that there 

 are almost no traces of the hypostoma in association with the large 

 number of specimens of the cephalon that occur at many localities, 

 both in Nevada and Alberta. The hypostoma of 0. canadensis is 

 unusually abundant in association with that species and O. gilberti. 



Observations. — In my earlier work [Walcott, 1884, 1886, and 

 1891] I gave a large variation to this species, and included in it 

 forms that are now grouped under Callavia nevadensis and Olenellus 

 fremonti. As now restricted, O. gilberti includes forms that have 



