176 F. R. Coujjer Reed — On the Phacopidce. 



nearly lobeless shoi't glabella, and the specialised types of Balmanites- 

 witli an elongated well-segmented glabella. Hoernes ' bad traced 

 the whole history of these two parallel branches of the family in 

 Bohemia from the Ordovician to the Devonian, and has clearly 

 shown that the typical Phacops is derived by a series of progressive 

 changes from the primitive forms of Dalmanites which form the 

 root of the whole familj'. 



Along the Dalmanites-line a pentamerous segmentation of the 

 head-shield with a more or less elongated cranidium and glabella 

 have persisted, though the glabella has been occasionally modified 

 by the unequal or irregular growth of the lobes, and even by the 

 partial obliteration of some of the lateral furrows ; the head-shield 

 also has in some cases accompanied these changes by the production 

 of various marginal or superficial ornaments or processes, but their 

 development in other cases has been independent. 



Along the other line there has taken place a general shortening 

 of the cranidium, with a loss of segmentation occasioned by the 

 obsolescence or disappearance of the lateral furrows of the glabella; 

 the lobes have become more or less completely fused, the glabella 

 inflated, and the outline of the head-shield generally rounded 

 without any marginal processes. 



Concurrent changes in the thorax and pygidium are also noticeable 

 along each line. 



In the Palmanites-hr?inch the thorax tends to become flattened, 

 and to develop free-pointed ends to the pleurge, while the number 

 of the segments in the pygidium increases, and its extremity 

 becomes mucronate, or the margin may be produced into various 

 spinose processes. The general tendency to develop spines and 

 processes on the head-shield and pygidium is a marked characteristic 

 of the later members of this branch. 



On the other hand, the P/mcops-branch tends to have all points 

 rounded ofi"; the genal angles of the head-shield are usually rounded 

 and not produced ; there is an absence of spinose ornaments on the 

 head ; the thorax becomes more convex ; the pleurje have their ends 

 obtuse or rounded ; the pygidium is not elongated, but assumes 

 a semicircular more or less transverse form without terminal or 

 lateral processes, and consists of comparatively few segments. 

 Hoernes traces the origin of this P/^acops-branch from the primitive 

 Dalmanites-forms (e.g., D. socialis, D. PhilHpsi, etc.), and upwards 

 through the Silurian group of Phacops Glockeri and its allies. 



As we should expect from our knowledge of the ontogeny, the 

 earliest members of the whole family have a pentamerously 

 segmented head-shield. This is the case in Bohemia, as Hoernes 

 notes, and also in England, where the earliest Phacopids are 

 Ph. Nicholsoni of the Skiddaw Slates and Ph. llanvirnensis of the 

 Upper Arenig (Llanvirn Beds). 



The primitive Dalmanites-gvoui^ may be therefore regarded as 

 constituting the stock from which later gi-oups have branched off, 



1 Hoernes, Jalub. Geol. Rciclisanst. Wieu, Bd. xxx, Ileft 4 (1880), pp. 651-686 ; 

 and id., Kosmos, Jahrg. iv, Bd. viii (1880), pp. 20-32. 



