RAYMOND AND BARTON: AMERICAN SPECIES OF CERAURUS. 527 



marked that we believe the groups worthy of generic names. Cheiru- 

 rus insignia Beyrich, has the characteristics of the second group, so 

 that group becomes the Cheirurus in our restricted sense, while the 

 first group is typified by Ceraurus pleurexanthemus, the type-species of 

 Ceraurus. Reed cited Cheirurus exsul Beyrich, as the type both of 

 the restricted genus Cheirurus, and of his subdivision of the restricted 

 genus in which the pygidium was cyrtometopiform. This species, 

 although described by Beyrich along with Bohemian trilobites, is a 

 northern form, and a Ceraurus. It was not described by Beyrich in 

 the paper in which he first described the genus Cheirurus, and it can- 

 not be taken as the type of that genus. 



The genus Cheirurus, as we understand it, comprises those species 

 of Reed's restricted genus Cheirurus which have the posterior pair of 

 glabellar furrows nearly straight, sloping more or less rapidly back- 

 ward, and opening directly into the neck-furrow at or near the axis 

 of the glabella; whose basal glabellar lobes are triangular, and not 

 separated by a portion of the glabella; and the pleural spines of 

 whose pygidia are subequal in length. 



Type. — Cheirurus insignis Beyrich. 



The genus Ceraurus, on the other hand, comprises those species 

 characterized by short posterior "glabellar furrows which slope only 

 gently backward and do not open directly into the neck-furrow but 

 are connected with it by longitudinal constrictions; whose basal lobes 

 are quadrangular, rather than triangular; whose glabella is usually 

 more or less strongly pustulose; and the spines of whose pygidium are 

 very unequal in length, the first pair being strongly developed, and the 

 remainder very short or absent. 



Type. — Ceraurus pleurexanthemus Green. 



Key to American Species. 



A. Glabella or neck-segment strongly spinose 



a. A pair of short horn-like spines on the crest of the frontal lobe 



bispinosus, sp. nov. 



b. A single median spine on the neck-segment . numitor Billings 



B. Glabella merely pustulose or with small spines 



c. Genal angles without spines 



Glabella expanding rapidly forward misneri Foerste 



d. Genal angles with spines 



A. Glabella expanding forward 



