F. B. Ooicper Reed — On the Phacopidce. 227 



tei-med Homalops ^ apparently belongs to this stage, and the final 

 result of the extraordinary growth of the first lateral lobes may 

 perhaps be found in Monorakos,^ in which the second and third ^ 

 lateral furrows are only represented by pits. A whole chain of 

 transitional forms showing intermediate conditions between Pterygo- 

 metopus and Chasmops can be traced, and there is no hard and fast 

 line between these subgenera. Hall & Clarke^ diverged widely 

 from the original conception of Chasmops, and erroneously inter- 

 preted the characters of the type-species by maintaining that the 

 great ' cat's ear ' lobes were formed by the coalescence and fusion of 

 the first and second lateral lobes on each side. The definition of the 

 subgenus given by these authors and the species included by them 

 in it cannot therefore be accepted. In all true members of Chasmops 

 the second lateral lobes are recognisable and show no coalescence with 

 the first lobes. In the American representatives of Pterygometopiis, \ 

 on the other hand, there is, as Clarke * has pointed out, a frequent 

 tendency for a coalescence of the first and second lateral lobes, and 

 the culmination of this structural modification seems to be met with 

 in MbnoraTcos. For Pterygometopus is by no means a homogeneous 

 group, as Schmidt himself has pointed out, and several subdivisions 

 in it may ultimately be recognised, though at present their limits 

 and relations are scarcely well defined. 



The connection between Pterygometopus and the other branches of 

 the Phacopidge has not at present been clearly traced, but its 

 relations are undoubtedly wide and varied. Clarke (op. cit.) 

 considers that a Trenton species (Dalmanites achates, Billings) is 

 a link between Dalmanites and Pterygometopus, but his view that 

 lobal coalescence in the glabella is a sign of immature development 

 does not commend itself to me. It may be noted that in many 

 respects, such as the tendency to augment the number of segments 

 in the pygidium and to develop spines on the genal angles, the 

 evolution of the Dalmanites and Pterygometopus branches has been 

 analogous and parallel. The relations of Pterygometopus to Acaste 

 ( = Phacopidella) are also worthy of remark, and Schmidt considers 

 that Pt. NieszTcowslci forms a passage over to the latter group. The 

 occasional partial loss or reduction of the second lateral furrow of 

 the glabella in Pterygometopus (e.g. Pt. Brongniarti) is paralleled by 

 certain species of Acaste (e.g. Ph. Downingice). 



Proposed Classification. 



The results of the above investigation into the relations of the 

 various groups, genera, or subgenera of the Phacopidee which have 

 been from time to time established show that the following classi- 

 fication may be suggested as based on phylogenetic principles : — 



1 Remele: Zeitschr. deut. Geol. Gesell., Bd. xxxvi(1884), p. 200. 



2 Schmidt: Bull. Acad. Imper. Sci. St. Petersbourg, xii, No. 4 (1886), p. 414. 



3 Hall & Clarke: Palsont. New York, vol. yii (1888), p. 63. 

 ^ Clarke : Lower Silur. Trilob. Minnesota, 1894, p. 732. 



