CEUSTACEA. 



Genus AaNOSTUS Brongniart. 



AONOSTUS INTERSTRICTA (sp. nov.) Head and pygidium of almost 

 exactly equal size and shape, and otherwise closely resembling each 

 other/ Head a trifle broader than long, regularly rounded in front ; 

 sides at the postero lateral regions subparallel ; postero-lateral angles 

 truncated; the whole exterior margin, including the truncated por- 

 tions just named, provided with a narrow, raised rim, forming a nar- 

 row, linear depression between it and those portions of the head which 

 it incloses ; space between this linear depression or furrow and the 

 glabella a little wider posteriorly than it is in front, convex, the surface 

 apparently smooth. Glabella conical, widest posteriorly, moderately 

 convex ; sides nearly straight, well denned by the dorsal furrows, sharply 

 rounded in front, a minute tubercle on the median line near the back 

 end, and a shallow furrow extending across near the front end. 



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

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

 its segments tumid at the ends where they reach the dorsal furrows ; 

 lateral lobes very narrow, pleurae about as wide as long, each pleura 

 tumid and rounded at its exterior end. 



Pygidium with its outline like that of the head, and provided also 

 with a similar raised marginal rim and furrow ; axial lobe a little 

 longer than the glabella, and consequently reaches a little nearer 

 the posterior margin of the pygidium. than the glabella does to the 

 anterior margin of the head, moderately convex ; sides curving a little 

 outward, provided with a minute tubercle on the median line near the 

 anterior end, corresponding with the one on the glabella; space between 

 the dorsal furrows and the marginal furrow convex, apparently smooth. 

 Upon the outer edge of the border of the pygidium, on each side, a lit- 

 tle nearer to the axial extremity than to the autero-lateral angles, there 

 is a minute protuberance suggestive of an incipient spine. 



Length of body, eight millimeters ; width of head, five millimeters ; 

 width of pygidium, same as head. 



Position and locality. Shales of the Primordial period at Antelope 

 Spring, House range, Utah. 



Genus OLENELLUS Hall. 



OLENELLUS GILBERT! Meek (manuscript).* Head subsemicircular 

 or semi-oval, the length being from one half to two-thirds as great as 

 the width ; both the external and posterior margins bordered by a nar- 

 row, continuous, slightly-raised rim, placed a little within the edge of 

 the margins; the postero-lateral angles produced into slender spines, 

 which are terete, not widened at the base, and, in the specimens, are 

 about equal in length to one quarter of the transverse diameter of the 

 head at its base. Near the postero-lateral angles of the head, the pos- 

 terior margin bends abruptly forward, forming a kind of notch or small 

 retreating angle with the backward-projecting spine, giving the outer 

 corner of the movable cheek the appearance of being rounded. Eyes 

 narrow, broadly arching outward, narrowness and convexity of curve 

 both increasing posteriorly ; their outer margins about equidistant from 



* The descriptions of this and the following species were written by Mr. Meek before my 

 own were, and now form a part of Mr. Gilbert's report to Lieutenant Wheeler, which is 

 awaiting publication. C. A. W. 



