THE NATURAL HISTORY SURVEY. 247 



Genus 10. - ARCTINURUS Castelnau, 1843. 



Glabella completely ' marked off by the dorsal furrows, 

 the median lobe extending to the occipital furrow, the first 

 and second lateral lobes combined and extending to the occipital 

 furrow, the third lateral lobes squeezed out so as to be entirely 

 absent; the second furrows obsolete or indicated by mere in- 

 dentations of the first furrows; occipital lobes absent. Pygidium 

 with one or two axial segments, the axis reaching about half 

 way to the posterior border, the broad post-axial region with 

 a sinuate or emarginate posterior margin ; three pairs of grooved 

 pleural segments, the two anterior pairs complete and with 

 free points, the posterior pair complete or nearly complete. 



Arctinurus occidentalis (Hall), pi. xx, figs. 10-12. 



Description. Pygidium somewhat pentagonal or hexagonal 

 in general outline, broader than long, the two lateral margins 

 sub-parallel, the anterior margin nearly straight or sloping 

 away laterally on each side of the axis. ' Axis strongly convex 

 in the anterior third of its length, with one distinct annulation 

 anteriorly and a second one a little less distinct and broader; 

 the greatest elevation is at the anterior margin from which 

 point the slope is abrupt laterally and more gentle posteriorly; 

 back of the elevated portion is a depressed post-axial region; 

 at the anterior margin the axis occupies about one-fourth the 

 total width of the pygidium, the axial furrows converge rapidly 

 posteriorly to a point just back of the elevated portion where 

 the width of the axial region is about one-third its width in 

 front, from here the axial furrows are slightly divergent pos- 

 teriorly to the posterior margin which is deeply sinuate between 

 the posterior extremities of the axial furrows which terminate 

 near the ends of. a pair of strong, acute, backwardly pointing 

 extensions of the pygidium. Pleura nearly flat throughout the 

 greater portion of their width, becoming slightly convex in 

 proximity to the anterior elevated portion of the axis, divided 

 into three pairs of deeply grooved segments which are strongly 

 bent backward, their terminations being acutely angular free 

 points, between which are deep emarginations, the terminations 

 of the six angular pygidial extensions are directed posteriorly, 

 the median pair, at either side of the posterior margin of the 

 axial regipn, extend the farthest back, the terminations of the- 

 second pair are a little farther forward, and the outer pair a 

 little in front of the second.. The surface of the pygidium is 



