F. R. Coicper Reed — Evolution of Cheirurus. 123 



Eeturning now to Cyrtometopus proper, we find it is succeeded by 

 Cheirurus (sens, str.), the characters of which have been already 

 given. The connecting link, so far as the pleurae are concerned, is 

 indicated by Cyrto. Zembtiitzki (Eichw. sp.), in which the pleural 

 groove has a slightly diagonal direction. In the characters of the 

 glabella and head-shield, Cyrto. Flautini (Schmidt) shows some 

 approach to Cheirurus (sens, str.), while Ch. yelasinosus (Portl. s-p.) 

 has nut the marginal furrow communicating with the axal turrow, 

 as it does in typical members of the subgenus Cheirurus (sens. str.). 

 In the pygidia of Cyrtometopus and this subgenus there are striking 

 points ot similarity, for the anterior pair of pygidial pleurae in many 

 species of Cheirurus (sens, str.) is greatly enlarged and elongated so 

 as to embrace the two posterior pairs, and the fourth pair is repre- 

 sented by a single median terminal piece. 



With regard to the pygidium, it is noticeable that the species of 

 Cheirurus (sens, str.) fall naturally into two groups when this portion 

 alone of the body is considered, viz. : (I) the species with Cyrto- 

 metopian pygidia as just described, of which an example is Ch. 

 exsul (Beyr.) ; 1 and (2) the species with three almost equal-sized 

 pairs of short pleurae, with a short single median or bifid piece 

 representing the fourth pair. Such a condition exists commonly in 

 Ch. bimucronatus (Murch. sp.), though many specimens from the 

 Wenlock Limestone show a more or less marked enlargement of the 

 first pair ot pleurae. 



As regards these pygidial characters, the Kussian species appear 

 to belong entirely to the first group; in Bohemia the second group 

 preponderates, as it does in Britain; while in Scandinavia the Russian 

 type is in a majority. The period to which Cheirurus (sens, str.) is 

 restricted extends trom the middle part of the Ordovician to the 

 upper part of the Silurian. Thus, while it commences during the 

 reign ot Cyrtometopus, it lives on after that subgenus has dwindled 

 away or disappeared. 



In Britain Cheirurus sg. is represented by Ch. bimucronatus 

 (Murch. sp.) — with its three varieties, bimucronatus and centralis of 

 Salter, and acanthodes (Marr and Nicholson) 2 — ranging from Bala to 

 Ludlow beds; Ch. yelasinosus (Portl. sp.), trom the limestone ot the 

 Chair of Kildare, on the same horizon probably as the Keisley Lime- 

 stone, from which come also Ch. ylaber (AngelinJ and Ch. keisleyeusis 

 (Reed MS.). Ch. cancrurus (Salter), which occurs in the Kildare 

 and Keisley limestones, may belong to this subgenus ; but though the 

 head figured by Salter a undoubtedly belongs to this subgenus, it is 

 very doubtful whether it is rightly associated with the extremely 

 atavistic pygidium on which the species was originally founded. 



1 A case of extreme specialization in this group is Ch. subulatus (Liiiuarssou), 

 in which the pygidium has only two pairs ot free pleurae. 



- Marr and Nicholson. The Stockdale Shales : Quart. Journ. Geol. Soc. 1888, 

 vol. xl, p. 722, pi. xvi, tigs. 7 and 8. 



s Salter. Mon. Brit. Triloh. : 1'al. Soc. 1864, p. 72, pi. v, fig. 16. 



(To be continued in our April Number.) 



