108 F. R. Goicper Reed — Sedgwick Museum Notes. 



granulated, with small tubercles interspersed. Faint traces on the 

 frontal lobe of the V-shaped mark found in Chasmops. 



Thorax (imperfectly known) about twice as long as head-shield ; 

 axis regularly semi-cylindrical, prominent, convex, with slightly 

 swollen lateral nodes on axial rings. Pleuree strongly arched, with 

 deep diagonal furrow; fulcrum situated about half-way out; extra- 

 fulcral part sharply bent down and slightly curved forwards ; 

 extremities of pleurse rounded. 



Pygidium semicircular to semi-elliptical, gently convex, with 

 regular margin. Axis prominent, convex, conical, tapering to 

 a pointed extremity, not quite reaching posterior margin ; composed 

 of 9-11 rings, of which only the first 5-6 are usually well marked. 

 Axial furrows straight, converging posteriorly at about 20°-25°. 

 Lateral lobes gently arched downwards on each side, showing 5-8 

 regular, slightly curved pleurae, dying out towards the margin and 

 each with weak median pleural furrow. Border smooth, not crossed 

 by pleuras, but not marked off on upper surface by marginal furrow. 

 Infra-marginal doublure (seen in casts) convex, of moderate width, 

 extending inwards to tip of axis. 



Dimensions. 



mm. 



Length of head-shield 18-20 



Width of head-shield 36-40 



Length of pygidium ... ... ... ... 13-23 



Width of pygidium 22-33 



Affinities. — The general characters of this species indicate a close 

 relationship to the Ordovician forms of Dalmanites (Barrande) ^ found 

 in Bohemia. Hoernes - has carefully pointed out the characters which 

 differentiate them from the typical Silurian and Devonian members 

 of this genus as understood by Barrande, and they constitute 

 a group which in one direction appears to lead to that of 

 D. Sausmanni of Stages F, G, and H, and in the other direction 

 to the typical forms of Pliacops, sens, str., of the same Stages. 



The chief points which distinguish this older group of Dahnanites 

 are the rudimentary or absent frontal border, the slight separation 

 of the frontal lobe from the rest of the glabella, the smallness 

 or absence of the genal spines, the rounded or scarcely pointed 

 extremities of the pleurae, and the fewer segments in the pygidium. 



In Ph. Robertsi the head-shield with its distinct frontal border, the 

 lobation and shape of the glabella, and small genal spines resemble 

 those forms leading on to the D. Hnusmanni group ; but the rounded 

 extremities of the pleurae and the non-mucronate, semicircular 

 pygidium connect it with the species which seem to be the 

 ancestors of the typical Pliacops group. D. socinlis, Barr. (and 

 its varieties), I). Deshayesi, Barr., and D. Ilaiolei, Barr. — all from 

 Stage D in Bohemia — show strong points of resemblance in the head- 

 shield, but D. Phillipsi, Barr. in the thorax and pygidium. It is 



' Barrande : Sj'st. Silur. Boheme, vol. i, p. 299 et seq. and p. 528 et seq. 

 - Hoernes, Jahrb. Geol. Reichsaust. Wieu, Bd. xxx (1880), Heft 4, pp. 651-686 ; 

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



