No. 21. — A new Genus of the Cheiruridae, with Descriptions of some 



new Species. 



By Donald C. Barton. 



The genus Cheirurus Beyrieh, has been divided into numerous sub- 

 genera or genera. Schmidt * restricts Cheirurus in a subgeneric sense 

 to those cheirurids having eleven thoracic segments, the inner part 

 of the pleuron divided by a diagonal furrow, and the glabella expand- 

 ing slightly forward. Reed 2 follows Schmidt, but goes further and 

 distinguishes Crotalocephalus Salter. Cheirurus insignis Beyrieh, 

 not Cheirurus exsul Beyrieh, as Reed gives it, 3 is however, the genotype 

 of Cheirurus. In a previous paper the necessity has been shown for 

 distinguishing between Cheirurus and Ceraurus. In the present paper 

 it is proposed to separate a new genus from Cheirurus. 



Cheirurus as represented by Ch. insignis Beyrieh, has nearly straight 

 posterior glabellar furrows that open on the neck-furrow at or near 

 the central axis of the glabella. The basal lobes are triangular and 

 are not separated by a portion of the glabella. The glabella is smooth 

 and expands forward. 



The posterior glabellar furrows of Ceraurus are less than one third 

 the width of the glabella in length and do not open directly into the 

 neck-furrow, but are connected with it by a longitudinal constriction. 

 The basal lobes are small and are separated by a portion of the glabella 

 that, in width, is at least one third of the total width of the glabella 

 at that point. The glabella and cheeks are pustulose and the gla- 

 bella expands forward. 



There is a group of species including Cheirurus icarus Billings, 

 Cheirurus povipilius Billings, Cheirurus ingricus Schmidt, Cheirurus 

 ornatus Dalman, and others, which have always been included in 

 Cheirurus in the restricted sense of Schmidt and Reed, but which 

 show certain constant differences. The posterior glabellar furrows 

 are straight, are about one third the width of the glabella in length, 

 and do not open into the neck-furrow. Their inner ends are connected 



i Revision der Ostbaltischen Silurischen trilobiten. Mem. Acad. imp. sci. St. 

 Petersburg, L881, ser. 7, 30, no. 1, p. 122. 



2 Evolution of Cheirurus. Geol. mag., 1896, dec. 4, 3, p. 117, et seq. 



3 Loc. cit., p. 117. 



