some British Fossil Crustacea. 411 
out the course of the eye-line in this genus, which separates it 
at once from Trinucleus, and renders it probable that the small 
tubercle in the middle of the cheeks in the 7. seticornis, T. fim- 
briatus, &c. are true eyes. The furrows at the base of the gla- 
bella also are distinctive for the genus*. 
Trinucleus gibbifrons (M‘Coy). 
Sp. Char. Cephalic shield nearly semicircular, length rather more 
than one-third of the width; glabella pyriform, rounded in 
front, gradually narrowing towards the base, compressed, ex- 
ceedingly gibbous, its height above the cheeks nearly equaling 
its width ; on each side of the neck-furrow (in casts) there is 
a deep puncture and another similar a little in front of it, a 
small spine on the middle of the neck-furrow ; cheeks sphe- 
rical triangles, height and width about equal, moderately con- 
vex; border of moderate width, three rows of punctures in 
front of the glabella, and five rows in front of the cheeks, more 
numerous at the sides, generally connected in front by radia- 
ting furrows, forming an imperfect fimbriation. Usual length 
of cephalic shield 3 lines. Surface very mimutely granulated. 
This very common species is figured without a name by Col. 
Portlock (Geol. Rep. pl. 1 B. f. 138 & 14). The fine granulation of 
the lobes of the head, and the extreme prominence of the gradually 
narrowing, pyriform, compressed glabella, separate this at once 
from either the 7. Caractaci or T. latus, with which it seems to 
have been confounded ; it is wider than the former, less so than 
the latter. From the two little punctures on each side of the 
base of the glabella, this strongly approximates the 7. scyllarus 
(His.) as distinguished from 7. seticornis; but although with 
abundance of specimens I cannot find an ocular tubercle or eye- 
line in the midst of the cheeks as in Tretaspis, to which those 
species belong ; those punctures indicate no doubt the existence 
of the muscles of the jaws and their appropriate rigs, but are 
not extended into transverse segmental furrows as in those last- 
named species ; in the radiation of the border and number of 
rows of pores in front it approaches shghtly the 7. radiatus 
(Murch.), but is distinguished by the head being rounded, the 
* The statement of Mr. Salter (Mem. of the Geol. Surv. vol. ii. pt. 1. 
p- 335), speaking of Hawle and Corda’s work, that ‘* Tetrapsellium is distin- 
guished from 7’rinucleus solely by a swelling in the axal furrow of the head ; 
it is almost identical else with 7. seticornis’"—might mislead the English 
reader with the idea that the present genus was identical with Tetrapsellium ; 
the fact is however, in his stricture on the Bohemian authors, Mr. Salter 
seems to have overlooked the grand character of their genus, namely its 
having but four body-rings (“ vier Leibringe,” H. & C. Monog. p. 42. 
8th line) ; it agrees otherwise with the common type of 7rinucleus, 
