OLENELLUS AND OTHER GENERA OF MESONACID.E 203 



into strong, long genal spines that are nearly as long as the thorax ; 

 the posterior border is broad, but not as convex as the frontal bor- 

 der ; it narrows toward the base of the glabella and shows a de- 

 cided tendency to curve witli a varying angle at the intergenal 

 angle [figs, i, 5, 6, and 10, pi. 29] ; the intermarginal furrow is 

 narrow and rounded inward on the sides and in front, and rather 

 more distinctly impressed within the posterior border. Glabella con- 

 vex, elongate, gradually expanding from the occipital segment to the 

 widest portion of the anterior lobe, dorsal furrow deep on the sides 

 and in front ; the anterior lobe is transverse, widest near its base, 

 gradually curving on the sides to the rather sharply rounded front 

 margin ; .the anterior and third pair of furrows extend from the 

 central third of the glabella obliquely forward and terminate at the 

 dorsal furrow ; the second pair terminate inside so that the second 

 and third lobes unite and enclose it [fig. 6] ; usually the space be- 

 tween the end of the glabellar furrow and the dorsal furrow is very 

 narrow, and it is often broken through [figs. 1,2]; occipital ring 

 separated from the glabella by two lateral furrows that are similar, 

 when the shell is not too much flattened, to the glabellar furrows ; 

 the occipital ring is broad, convex, and with a long, strong median 

 spine that curves backward over the axial lobe of the thorax to about 

 the sixth segment ; the base of the spine is strong and, in large speci- 

 mens, extends nearly across the occipital ring. The cheeks arch 

 up from the intermarginal furrow to the base of the eye, broadening 

 back of the eye and narrowing toward the front margin so as to 

 form only a narrow space of slightly variable width in different 

 specimens between the glabella and the intermarginal furrow. The 

 palpebral lobes are narrow, elongate, and gently arched outward 

 and backward from the dorsal furrow beside the posterior lateral 

 margin of the first glabellar lobe ; they terminate a short distance 

 from the dorsal furrow opposite the occipital furrow, thus giving 

 only a slight divergence between their anterior and posterior ends ; 

 they are elevated to about the same height as the center of the 

 glabella and slope rapidly into the depressed interpalpebral area, 

 the drop to the cheeks is very abrupt, and gives only a narrow space 

 for the visual surface of the eye. The only trace observed of a 

 facial suture is an elevated line on the cast of the interior of a 

 flattened cephalon ; the line starts at the posterior end of the palpe- 

 bral lobe and extends backward a short distance before curving out- 

 ward toward the marginal rim, which it crosses obliquely at about 

 the same place as in Callavia hroggcri [pi. 27, fig. i]. 

 5— w 



