464 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



frontal lobe by its smoothness and by being- nearly flat; posterior lateral 

 limb of fixed cheeks strong and extending obliquely outward and backward 

 from the base of the palpebral lobe; posterior rim narrow. 



Surface slightly granular under strong magnifying power. 



Formation and locality: Middle Cambrian, in bluff south side of 

 Pebble Creek, north of saddle to Soda Butte Creek, Yellowstone National 

 Park. 



SOLENOPLEURA Angelin. 



Solenopleura! weedi. 



PI. LXV, figs. 9, 9«, 



Of this species, only the central portions of the head occur in the col- 

 lection. The entire individual attained considerable size, as the largest 

 head has a length of 20 mm. The characteristic features of the head are 

 also shown in heads 9 mm. in length. 



The glabella is obtusely conical, having a width at the base in the 

 largest specimen of 10 mm., and a length from the center of the occipital 

 furrow to the frontal limb of 13 mm. It is separated from the strong 

 rounded occipital ring by a relatively broad, well-defined furrow. The 

 posterior pair of glabella furrows is indistinctly shown by very shallow 

 grooves in the case of the largest individuals; on the smaller individuals it 

 is not shown, except by a smooth spot. The glabella is separated from the 

 fixed cheeks and frontal limb by a well-marked dorsal furrow, and as it is 

 quite convex it stands out in clear relief from the general surface of the 

 head. Frontal limb short, 1.5 mm. in length in the large specimen. It is 

 separated from the strong rounded frontal rim by a well-defined groove; 

 laterally it passes into the free cheek, which is of medium width. The pal- 

 pebral lobe is of medium size and situated at a point opposite the transverse 

 central line of the glabella. A faintly defined occular ridge extends abruptly 

 backward across the fixed cheek from a point a little back of the front of 

 the glabella to the front angle of the palpebral lobe. It is defined more 

 by the presence of a slight groove in front of it than by the elevation of the 

 ridge itself. The posterior lateral limb of the fixed cheek extends outward 

 two-thirds of the width of the glabella at the base ; it is separated from the 

 posterior rim by a well-defined furrow. 



