PEIMORDIAL PERIOD. 39 



Head a tiifle broader than long, regularly rounded in fi-ont ; sides at 

 the postero-lateral regions subparallel; postero -lateral angles truncated; 

 the whole exterior margin, including the truncated portions just named, pro- 

 vided with a narrow, raised rim, the elevation of which forms a linear 

 depression, or groove, between it and those portions of the head which it 

 incloses ; space between this marginal depression and the glabella a little 

 wider posteriorly than it is in front, convex throughout, and its smface 

 apparently smooth. Grlabella conical, widest posteriorly, moderately con- 

 vex, sides nearly straight, well defined by the dorsal furrows, abruptly 

 rounded in front ; a minute tubercle situated on the median line near the 

 posterior end, and a shallow groove or furrow extending across near the 

 front end, defining a frontal lobe of moderate size. 



Thorax narrower than the head and pygidivim, giving the body the 

 appearance of being constricted at the middle ; axial lobe broad, consisting 

 of two segments, both of which are tumid at the ends adjoining the dorsal 

 fun'ows ; lateral lobes very narrow ; pleurae almost as wide as long ; each 

 pleura tumid, and rounded at its exterior end. 



Pygidium having an outline like that of the head, and is also provided 

 with a similar elevated marginal rim and linear depression within it ; axial 

 lobe a little longer than the glabella, and consequently that lobe reaches a 

 little nearer the posterior margin of the pygidium than the glabella does to 

 the anterior margin of the head, moderately convex in elevation and also in 

 each lateral outline ; a minute tubercle is situated on the median line near 

 the anterior end, corresponding in size and relative position with the one on 

 the glabella before mentioned ; space between the dorsal furrows and the 

 margin convex, its surface apparently smooth ; upon the outer edge of the 

 border of the pygidium, at each side and a little nearer to the axial extremity 

 than to the antero-lateral angles, there is a minute protuberance, suggestive 

 of an incipient spine. Besides the slight differences between the head and 

 pygidium, already referred to, the pygidium differs also in having a faint 

 appearance of segmentation of its axis and in a slight folding-backward of 

 the marginal lim at the antero-lateral angles. 



Length of body, eight millimeters ; width of head and also of the 

 pygidium, five millimeters ; width of thoi'ax, four millimeters. 



