226 F. R. Coivper Reed— On the Phacopidce. 



the possible exception of one or two South American^ and one 

 South African species which are found in the Devonian-), it seems 



\ I to deserve a distinctive subgeneric name, and that of Fhacojnde-lla 

 is here suggested ; type, Ph. Glockeri, Barr. 



' The final and most highly specialised stage of this branch is found 

 abundantly in the D evonia n period in Europe, America, and Asia, but 

 it appears in the higher part of the Silurian in all three continents 

 and also in Australia.^ It differs only from the previous stage in 

 the greater accentuation of the above-mentioned characters. The 

 glabella tends to become more enlarged, inflated, and transverse ; the 

 pentamerism and segmental furrows become very faint or are quite 

 lost ; the lateral lobes of the glabella (excepting generally the 

 nodular basal pair) are inseparably fused ; the genal angles and 

 extremities of the thoracic pleurte are invariably rounded ; the 

 pygidium is semicircular or transverse, composed of very few 

 segments, and has a simple regular border. There is a complete 

 absence "of spinose ornaments. The eyes in typical FJiacops are 

 large, but they may be reduced in size or absent {Trimerocephaliis) . 

 Gurich's^ distinction between Trimerocephalus and the typical 

 Phacops seems quite insufficient and arbitrary. 



This final stage may have the old name Somatrihelon ^ revived for 

 it, with Phacops rana, Green, as its type. 



Branch C. 



Lastly, there is the somewhat distinct and disconnected branch of 

 the Phacopidge of which the earliest types are included in the sub- 



vy genus Pterygometopus.^ This subgenus appears to be confined to 

 Northern Europe and North America, as Freeh has pointed out, and 

 jit is characterised by the pentamerous lobation of the head-shield, 

 as we should expect in an early representative of the family, for 

 Pterygometopus belongs especially to the lower part of the Ordovician. 

 But there is a constant teudency for the unequal growth of the 

 lateral lobes by the first lobe being developed at the expense of 

 the others; and as the second and third lobes get correspondingly 

 i-educed in size by the hypertrophy of the first lobe there is an 

 insensible passage into the condition which marks the subgenus 



^ Chasmops,'' in which the second lobe is almost squeezed out or 

 represented by a mere tubercle. The indifferently defined group 



' Ulricli : Neues Jahrb. f. Miner., Beil. Bd. viii (1893), p. 21 [Acaste devo?iica). 

 Clarke : Arcliiv. Mus. Nac. Eio de Janeiro, vol. ix (1890), pp. 15-17 (P/i. brasilie>isis). 



2 Lake: Ann. S. Afr. Mus., vol. iv, pt. 4 (1904), No.. 9, p. 205 (PA. africanus). 



3 Etherid2;e & Mitchell : Proc. Linn. See. New South Wales, ser. ii, vol. x 

 (1895), p. 486. 



* Glirich : Verhandl. russ. kais. Miner. Gesell. St. Petersburg, ser. ii, Bd. xxxii 

 (1896), pp. 359, 362. 



5 McMurtrie : " Sketches of Louisville and the Fall of the Ohio," p. 74 (Louis- 

 ville 1819). The tj'pe-species of Somatrikelon [S. megalomaton, McMurtrie) has 

 been identified with Fhacops rana, Green (Vogdes, Bibliogr. Pala^oz. Crust., Occas. 

 Papers Calif. Acad. Sci., No. 4, 1893, p. 163). 



•' Schmidt: Eev. Ostbalt. Silur. Trilob., Abth. i (1881), p. 62. 



' McCoy: Ann. Mag. Nat. Hist., ser. ii, vol. iv (1849), p. 403. 



