F. R. 0. Reed—On the genus Trinucleus. 119 
by a nearly straight oblique line along which the surface is bent or 
angulated. The semilunar outer band thus formed slopes down more 
or less steeply to the fringe, and is differentiated from the rest of the 
genal area. The line of angulation is marked at its posterior end by 
a short ridge forming its crest, beyond which lies a deep pit in the 
pleuro-occipital furrow. Salter’ considered that this line of angu- 
lation occupied the place of the facial suture of other trilobites. 
The semilunar outer band is smooth in the species 7. Murchisont, 
but the whole of the inner portion (with the exception of a narrow 
region alongside the basal part of the glabella) is covered with 
a coarse reticulation of fine raised lines which arise as a fan-like 
group of thicker lines radiating from a small notch behind a tubercle 
projecting into the axial furrow at the level of the first lateral furrow 
of the glabella. These radiating lines rapidly break up into the 
general reticulation of the surface, so that the radial arrangement is 
almost lost. The pseudo-frontal lobe of the glabella, which possesses 
a median tubercle, has similar fine reticulation, but anteriorly the 
meshes are elongated transversely so as to be roughly concentric to 
the front end; posteriorly they became hexagonal or polygonal lke 
those on the genal areas. The small inner portion of the genal area 
on each side of the base of the glabella is only minutely granulated, 
which is significant (see below). 
In Z. Gibbst, Salter, and 7. Etheridge, Hicks, the angulation of 
the genal areas and differentiation into two portions exhibit the same 
general characters. But in the former species the angulation is 
sharper and the outer band narrower, and the fan-like arrangement 
of the reticulating lines on the posterior part is not so clear. 
In 7. Htheridgei the genal areas are divided into two nearly equal 
parts by the angulation, but itis less sharply marked and often nearly 
obsolete. The different superficial characters of the two parts are, 
nevertheless, retained. No definite point of origin for the reticulations 
‘ean be recognized in this species, a general honeycomb-like meshwork 
existing all over the inner region with the exception of the inner 
posterior angle alongside the base of the glabella, which is smooth. 
A median tubercle is usually visible on the pseudo-frontal lobe of the 
glabella. In none of these three species is there a definite ocular 
ridge or ocular tubercle on the genal areas. 
It appears probable that we may correctly compare the radiating 
and reticulating lines on the surface of the genal areas with the 
so-called nervures or veins on the cheeks of Dionide (although their 
radiating arrangement is better preserved in the latter genus), for 
they arise from the same point on the axial furrows. Similar 
structures having an identical origin are found in the cheeks of many 
Cambrian genera, which are devoid of compound eyes, e.g. Hrinnys, 
LElyx, Liocephalus, and other members of the Conocoryphide. 
Beecher regarded the meshwork and lines as belonging to the 
nervous system, but Lindstrém? considered them as ramifications of 
the circulatory system, the larger lines being the main vessels and 
1 Salter, Mem. Geol. Surv., vol. iii, 2nd ed., p. 516, 1881. 
2 Lindstrém, Kongl. Svensk. Vet. Akad. Handl., Bd. xxxiv, No. 8, 
pp. 18-20, 31-3. 
