526 bulletin: museum of comparative zoology. 



by a diagonal furrow, the glabella expands forward, and the lateral 

 furrows of the glabella are always distinct. Reed x follows Schmidt 

 in this restriction of Cheirurus, but goes further and separates Salter's 

 subgenus Crotalocephalus from the group which was included in 

 Cheirurus by Schmidt. Reed divided Cheirurus as restricted by him 

 into two groups, the first with w 7 hat he designated as the cyrtometo- 

 pian pygdium (essentially like that of Ceraurus), and the second with 

 the Cheirurus insignis type of pygidium, that is, with three pairs of 

 subequal pleural spines, and a short median spine. When the char- 

 acteristics of these two groups are studied, it is found that they differ 

 in the structure of the head as well as of the pygidium. In the second 

 group the posterior pair of glabellar furrows are nearly or quite straight, 

 slope backward at a considerable angle, and open directly into the 

 neck-furrow at or near the middle of the glabella. The basal lobes 

 thus cut off are triangular, and their inner ends almost meet. In this 

 same group the surface of the glabella is smooth, and while the cheeks 

 may be pitted, they are not pustulose. It is further noticeable that the 

 glabella is large in this group, usually occupying one third or more 

 of the width of the cephalon. In the first group, with crytometopian 

 pygidium, the glabella is usually (in the American species always) 

 pustulose, and the glabella is usually less than one third the total width 

 of the cephalon. The posterior glabellar furrows are short, each 

 extending only about one third the distance across the glabella, and 

 instead of connecting with the neck-furrow, their course is approxi- 

 mately parallel to it. There is, however, usually a secondary constric- 

 tion parallel to the long axis of the glabella, which connects the inner 

 end of the furrow with the neck-furrow. This constriction may be 

 very faint or very deep. In the latter case the posterior glabellar 

 lobes are completely isolated, are nearly square in outline, and are 

 separated from each other by a portion of the glabella whose width 

 is about equal to that of one of the posterior lobes. All of the known 

 species of the first group are found in strata of Ordovician age, while 

 those of the second occur in the upper Ordovician (Bohemia only, so 

 far as knowm) and Silurian. In geographical distribution, only the 

 first group is found in Russia, and it preponderates in Scandinavia. 

 The home of the second group is in Bohemia, but it is also more promi- 

 nent than the first group in Great Britain. Both groups are present 

 in North America. 



The differences between these two groups are so constant and well 



1 Evolution of Cheirurus. Geol. mag., 1896, dec. 4, 3, p. 117. 



