224 F. R. Coicper Reed — On the PhacopidcB. 



VII. — The Classification of the Phacopid^. 



By F. E. CowPER Eeed, M.A., F.G.S., of the Sedgwick Museum, Cambridge. 



( Concluded from p. 178.) 



IN North and South America and in South Africa ^ there is 

 a considerable variety of types of the Da?ma7ii7es-branch in the 

 Devonian period which have in many cases received distinctive 

 subgeneric names ; but species retaining the typical characters of the 

 Silurian forms persist at any rate in North America. Many of these 

 Devonian forms show incomplete second lateral furrows on the 

 glabella, these furrows not reaching the axial furrows and causing 

 a partial coalescence of the two middle lateral lobes. This tendency 

 towards the fusion of the first and second lateral lobes of the glabella 

 is a departure from the perfect segmentation found in typical 

 Silurian members of Dalmanites, and has caused Clarke ^ to group all 

 such forms together into the subgenus or section SynpTioria. This 

 type of structure, as Van Ingen ^ has recently shown, is not unknown 

 amongst the Silurian species of Dalmanites in America, but it finds 

 its most pronounced development in Devonian time and occurs in 

 the groups Coronura, Corycephalns, Odontocephalus, and Probolium, 

 all of which are put by Clarke in the section SynpTioria. The 

 marginal ornamentation and diff'erent processes on the pygidium and 

 head-shield on which these four groups have been founded are 

 scarcely of the same structural importance as the modifications of the 

 glabellar segmentation. As in other families, the spinosity of 

 these forms is the symbol of a last expiring effort before extinction. 



Summarising the above facts and deducing therefrom the obvious 

 conclusions, we see that the Z)a/manties-branch falls into three 

 divisions, which chronologically succeed each other and mark 

 progressive stages of development. Broadly speaking, they may be 

 said to characterise respectively the Ordovician, the Silurian, and 

 the Devonian. 



Division I. Primitive and composite forms (e.g., D. Phillipsi, 

 Barr., D. socialis, Barr., etc.), with pentamerism of head well 

 marked ; lobes of glabella distinct ; frontal lobe not strongly 

 detached ; frontal limb rudimentary or absent ; genal angles 

 rounded or shortly pointed ; pleurae of thorax with rounded or 

 pointed ends ; pygidium composed of few segments, rarely more 

 than ten, usually of rounded outline. (Subgen. prop. Dalmanitina ; 

 type, D. socialis, Barr.) 



Division 11. Typical forms (e.g., D. caudatus, Briinn,, the type of 

 Dalmanites, sens, str.), with pentamerism of head well marked ; 

 frontal lobe of glabella detached ; frontal limb well developed ; 



^ Lake : Annals of the South African Museum, vol. iv, pt. 4, No. 9 (1904), 

 pp. 203-213. 

 - 2 Clarke, "Low. Silur. Trilob. Minnesota" : Rep. Geol. Nat. Hist. Surv. Minn., 

 vol. iii (1894), p. 732. 



2 Van Ingen : School of Mines Quarterly, Columbia University, vol. xxiii, Nov^ 

 1901, No. 1, p. 67. 



