Clause — Crustaceans from the Chemung Group. 



System. Uebersicht, PL VI, fig. ; B. meridioiialis, Ti*om. and Grasset, 

 Barrois's figure, talc, a Polypiers de Cabrieres, Plate I, figure 2a< 

 />'. ih/ysanopeltis, var. Waldschmidti, \. Koenen, Waldschmidt's figure, Zeitschr. 

 d. (I. Geo! GeselU-h., Vol. XXXVII, PL XXXVIII, fjg. 2; senesoens, etc., 

 etc.). This feature appears to be one which, irrespective of other variations 

 in structure, lias gradually passed through the change noted, and it is, 

 naturally, to lie observed that lower Devonian faunas embrace species, some 

 with the wide, some with narrower glabella, while thereafter, the earlier 

 type of glabella has disappeared. 



With reference to the characters of the pygidinm in this genus it may be 

 remarked that the only really useful, tenable subdivision of the group is 

 founded upon the presence of spinules upon the pygidial margin. This is 

 Thyanopeltis, one of the names introduced by Corda and designed by its 

 author to include species of this character; it has proven to possess a definite 

 stratigraphic value. In matter of outline there is considerable variation, from 

 the subsemicircular shape in the earliest species like B. lunatus, Bill., of the 

 Trenton, B. laticauda, Wahlenberg, of the lower Silurian of Sweden, and B. 

 hibemicus, Portlock, of the Caradoc, to the elongate-elliptical curve presented 

 ]>v the species of the Devonian. In this feature ]>. senescens is extreme, 

 surpassing the elongate, narrowing tail-plate of B. Kielcensis, Giirich, of the 

 lower upper Devonian (Cuboides horizon) of Poland. Corda (Prodrom einer 

 Monographic dei- bohm. Trilobiten, pp. 58, 59, 1847), attempted a further 

 division of the genus on the basis of the simplicity or duplication of the 

 median rib of rhe pygidium, proposing- for such species as show a bifurcation, 

 the name Dicranactis, and for those in w hich it is simple, Hohmeris. It has 

 long been evident, and is shown in the species in hand, that the duplication 

 of this rib is of such minor significance that it can be regarded only as an 

 individual character and we can not safely infer values from this trait. 



Barrande observed (Systeme Silurien, Vol. I, p. 840) that a division of 

 the species of Bronteus might be founded upon the number of ribs on the 

 pygidium, which are either six, seven or eight on either side of the median 

 rib. By far the greater number of species possess seven ribs, while six such 

 ribs are present only in the earliest species, e. </., B. hmabus, 11. hibernicvs, 

 B. laticauda. Herein B. senescens, having seven ribs, perpetuates the struc- 

 ture of the normal representatives of the genus. 



The definite time value of the subgenus ThysanopeUtS has been set forth 

 by Barrois, Ivayser, Freeh and other writers. The only representative of this 

 division yet known in American faunas is the species JJ. tullius, from the 



