F. R. Coicper Heed — Evolution of Cheirurus. 163 



a nearly obsolete line of puncta or with a plain surface, likewise are 

 preliminary to the smooth pleurae of Sphmrexochus. The presence of 

 a distinct fulcrum is an advance on Nieszkoioskia and the non- 

 fulcrum -bearing pleurae of Eccoptocheile. The four pairs of pleurae 

 to the pygidiurn with free ends are reversionary, and again point to 

 the independent evolution and modification of the pygidium. In 

 England the only species so far recognized are Ps. conformis (Ang.) 

 and Ps. subquadratus (Eeed MS.) from the Keisley Limestone, and 

 Ps. moroides (Marr and Nicholson) from the Stockdale Shales. 1 



A small group of species, of which Ch. octolobatus (Salter) is an 

 example, stands apart from those above described, by a combination 

 of characters showing widespread and complex affinities. The 

 smooth pleurae and the glabellar furrows resemble those of Pseudo- 

 sphcerexochus, while the very small free cheeks and position of the 

 facial suture ally Salter's species ( Ch. octolobatus) with Nieszkowshia. 

 But several features of the head-shield, pleurae, and pygidium seem 

 to link it to the Bohemian species Ch. neglectus (Barr.) and Ch. com- 

 pletus (Barr.) of Dd 5, and through them to the transitional forms 

 between Eccoptocheile and Cyriometopus. 



The relationship of Ch. trispinosus (Young), of the Penkill mud- 

 stones of Girvan and the Silurian of Gothland, is doubtful ; but its 

 discoverer, Prof. Young, suggested that it might be necessary to create 

 a special genus for its reception, 2 and Lindstrom 3 has recently done 

 so, and placed it in a new genus, Youngia. along with two new 

 Gothland species — Y. inermis (Lindstr.) and Y. globiceps (Lindstr.). 



The American species of Cheirurus have not been taken into 

 account in the foregoing sketch of the affinities and evolution of the 

 species of the genus, because specimens of them are not sufficiently 

 common nor their characters adequately known in this country to 

 admit of speculations of any value. 



Distribution in Space. 



The theory of the existence of North and South European provinces 

 for trilobites, at any rate during a great part of Lower Palaeozoic time, 

 seems to rest on a considerable accumulation of evidence, and in the 

 case of Cheirurus we can clearly perceive several facts in its support. 



Firstly, we find that the Cyrtometopus group attained a greater 

 development in the north of Europe than in the south ; its species 

 were much more numerous and its diversity of form much greater. 



Secondly, we find that southern Europe was characterized by the 

 absence of the northern forms Nieszkowshia, Pseudosphcerexochus, and 

 Sphcerocoryphe. 



Thirdly, we find that the Cheirurus (Eccoplo.) claviger group is 

 confined to the south. This group of species contains the earliest 

 members of the genus in Bohemia, and in that country alone a fairly 



1 Quart. Journ. Geol. Soc, vol. xliv, 1888, p. 722, pi. xvi, fig. 9. 



2 Young. Proc. Nat. Hist. Soc. Glasgow, 1868, I, pt. i, p. 171, T. i, figs. 4, 6b. 



3 Lindstrom. Fortecknins? pa Gotland Siluriska Crustaceer, in Ofversigt Kongl. 

 Vet. Akad. Forhandl. 1885, No. 6, p. 50, T. xiii, figs. 11, 12. 



