F. R. Cowper Reed — Evolution of Cheirurus. 117 



iv. woodwardian museum notes. 



Notes on the Evolution of the Genus Cheirurus. 



By F. R. Cowper Reed, M.A., F.G.S. 



rPHE genus Cheirurus offers an interesting study in evolution. 

 J_ Barrande, in his monumental work on the Silurian System of 

 Bohemia, 1 drew up a classification of the genus based principally 

 on the character of the pleural grooves. Thus he divided the species 

 into two sections — 



Section I. Pleural groove parallel to the borders and slightly 

 marked. 10-12 segments to the thorax. 



Section II. Pleural groove oblique and deep. 11 segments to the 

 thorax. 



The first section he subdivided into three groups according as the 

 species had 10, 11, or 12 thoracic segments. The second section he 

 split into two groups: (1) with glabellar side-furrows not united 

 on the axis ; (2) with glabellar side-furrows united on the axis. 

 This last group corresponds with Salter's subgenus Crotaloceplialus. 

 Another provisional group was made to contain several forms whose 

 position could not be then established. 



The restriction of Section I to the Lower Silurian and of Section 

 II to the Upper Silurian and Devonian is also commented on by 

 Barrande, as well as the almost complete limitation of the second 

 group of Section II to the Devonian. The relationships of the 

 various species are also minutely discussed ; and on plate vi of the 

 Atlas there is given the complete series of modifications in the 

 pleurae between the extreme terms represented by Ch. claviger of 

 Section I and Ch. gibbus of Section II. 



Schmidt 2 has more recently discussed the affinities of the various 

 species and divided the genus into two sections, the first of which 

 is characterized normally by the possession of eleven thoracic 

 segments, each pleura being divided by a constriction into an inner 

 and an outer portion. The inner portion is marked by a longi- 

 tudinal or a diagonal furrow, and is also possessed of a narrow 

 articulating band along its anterior and posterior bordei - s, which 

 terminates at the constriction in a recurved, swollen, hook-like 

 process. A similar band with hook-like process is found on each 

 side of the axial portion of the head-shield along its posterior 

 border. To this section belong Cheirurus (sens, str.) and Cyrto- 

 metopus. These two forms mark two groups. 



Group 1 of this section of Schmidt consists of Cheirurus 

 (sens, str.) (type Ch. exsul, Beyr.). All the species have a diagonal 

 furrow on the inner portion of the pleura, and the outer portion 

 is swollen at its base. The marginal furrows of the head-shield 

 are always distinct, and run into the axal furrows without inter- 

 ruption ; the latter diverge more or less in front. The glabella 



1 Syst. Silur* Bobetne, vol. i: Trilobites (1852), p. 770; and Suppl. to vol. i 

 (1872). 

 - Revis. d. Ostbalt. Silur. Trilob. 1831, pt. i, p. 121. 



