Dr. F. R, Coivper Reed — The genus Homalonotus. 269 



transverse and nearly straight special suture on the upper surface of 

 the head-shield. It is not clear if Hall regarded this "frontal 

 suture" as a new independent commissure originating by itself, but 

 he apparently believed tlie facial sutures did not meet on the. upper 

 surface of the head-shield, but that they were directly continued by 

 the epistomal sutures extending on to the inferior surface. 



With regard to the existence of any distinctive features of this 

 section, the structure of the epistomal doublure resembles Tr. delphino- 

 cephalus, and the thorax seems almost identical ; but, as Hall (op. cit., 

 p. 10) remarks, Dipleura differs from the Homalonoti of the earlier 

 Devonian and Silurian of America and Europe in the obsolescence 

 of the annulations of the pygidium at maturity. Tlie hypostome is 

 subquadrate, with the posterior margin broadly excavated, and is 

 much like that of Tr. delplmiocephahis. The glabella has three pairs 

 of lateral furrows, which become obsolete at an early stage of growth. 

 As regards the structure of the head -shield and the flattened anterior 

 junction of the facial sutures, we may compare H. rhenamis, Koch,^ 

 and IT. ornatns, Koch,' but as regards the obsolescence of the pygidial 

 axis we see an approach to H. lavtcauda, Quenst.,' though the head- 

 shield and outline of the pygidium are distinct in that species. 



Kayser, in a footnote to Koch's paper (op. cit., p. 10), rightly 

 points out that the llhenish Devonian species JT. crassicauda, Sandb., 

 and TT. Ahrendi, Roem., mentioned by Salter under Dipleura, do not 

 strictly belong to this group on account of their strongly ribbed 

 and acuminate pygidia, but that H. Schusteri, Roem.,'' may be 

 referred to it. 



4. Brongniartia, Salter, 1865. 



The definition of Brongniartia given by Salter ^ is as follows : 

 "Depressed, with broad rounded head, remote eyes, well-defined 

 lobeless urceolate glabella, and many -ribbed rounded tail." 



Two divisions were established by Salter with the following brief 

 summary of characters: "(1) Rody scarcely trilobed ; the axis 

 broad {H. hisiilcatus is the type of the subgenus and of this section) ; 

 (2) body strongly trilobed; the axis narrow (type, H. rudis; this 

 leads off directly towards Calymene).'''' 



Refore discussing the characters and value of this subgenus proposed 

 by Salter we must remark that it is unfortunate that he chose the 

 preoccupied name Brongniartia. For Leach in 1824 used it for a genus 

 of Coleoptera, and Eaton* in March, 1832, proposed it for a new genus 

 of Trilobites, of which his Brongniartia carcinodea'' was choisen as 

 the type. The latter species, however, is now considered as identical 

 with Green's Triarfhrus becki. Eaton in June of the same year 



^ Koch, op. cit., p. 32, pi. iii, figs. 1-3. 



I Ibid., p. 23, pi. ii, figs. 1, 2. 



' Koch, op. cit., p. 55, pi. viii, fig. 4. 



* Eoemer, Beitr. z. Kennt. Nordwest Harz., iii, t. iii, fig. 20, 1855. 



^ Salter, op. cit., p. 104. 



® Eaton, Amer. Journ. Sci., ser. i, vol. xxii, p. 165, 1832. 



'' Eaton, Geol. Text-book, 2nd ed., June, 1832, p. 33, pi. i, fig. 3. 



