122 F. R. C. Reed—On the genus Trinucleus. 
portions of the ocular ridge mark a more advanced stage in develop- 
ment. To this question we will return later. With regard to the 
function of this structure Beecher (op. cit.) remarked that “from 
the direction of the optic nerve in Zimulus and its relations to the 
surface features of the cephalothorax, the eye-line [ = ocular ridge! 
probably represents the course of that nerve”. The Bohemian 
examples of 7. Bucklandt, according to Barrande, do not possess an 
ocular ridge, the ocellus being isolated on the genal area. 
It may be suggested that the ocular ridge in this Group 2 with its 
outward extension represents a specialized and concentrated nerve- 
supply and corresponds to a specially enlarged nervure of the diffuse 
radiating bundle of nervures in Group 1. Its presence is probably 
correlated with that of the ocelli; and the absence of visual organs 
in Group 1 seems to be connected with the breaking up and irregular 
extension of the nerves over the whole surface, since we find a similar 
condition in many blind Conocoryphide. Inthe large Girvan examples 
of Z. Bucklandi in which the ocular ridge is obsolete the reticulating 
lines which cover the genal areas with a polygonal meshwork start 
as radiating or sinuous nervures from the same point on the axial 
furrows as the ocular ridge and as the corresponding nervures in 
Group 1. ‘The extension of the ocular ridge to the postero-lateral 
angles which, as above-mentioned, occurs in several species may 
likewise be the concentration of the nerve-supply which in Group 1 
and 7. fimbriatus is split up into a convergent group of fine lines. 
The phylogenetic significance of these structures is discussed below. 
A few remarks may here be made on the puzzling variations in the 
development of the meshwork on the genal areas which is found in 
this second group. The surface in some cases is covered with the 
reticulating ornament, but in other cases appears smooth or punctate ; 
and internal casts likewise differ much in their appearance. These 
peculiarities amongst individuals of the same species or members of 
the same group are due to the existence of more than one layer in 
the shell and to the difference in the characters of the outer and inner 
surfaces of these layers. 
It seems that there are two layers to the shell, the outer one of 
which has an externally smooth or punctate surface, and is usually 
very thin and easily abraded, so that it is frequently missing from 
even well-preserved specimens. ‘The second or inner layer bears on 
its upper surface (which is in contact with the lower surface of the 
outer layer) the raised lines of the reticulating nervures, while the 
lower surface of the inner layer is only minutely granulated or smooth, 
the interlaminar nervures rarely showing through it. When, how- 
ever, this inner layer is thin and the nervures stout and strong, we 
find the reticulations showing through, and therefore impressed on 
the internal cast of the head-shiel ld, the ‘hexagonal or polygonal cells 
being represented as shallow pits, as can be seen in casts of 
Te Bucklandi from Girvan or of 7. seticornis from various English 
and Welsh localities. The nervures do not seem usually to project 
as raised lines on the inner surface of the inner layer; and the 
ocular ridge and tubercle are certainly hollow, for these structures 
show respectively as a groove and a pit on internal casts. When 
