F. R. C. Reed—On the genus Trinucleus. 121 
here, is of interest from possessing two well-developed raised nervures 
on the genal areas, starting from a point on the axial furrow alongside 
the pseudo-frontal lobe, and then diverging as they cross the genal 
areas to converge and unite a little in front of the posterior lateral 
angle. These structures are undoubtedly of the same nature as those 
described in 7. Murchisont and 7. fimbriatus, having the same point 
of origin, same general course, and same point of termination; and 
they bear a specially close resemblance to the nervures on the cheeks 
of Dionide Lapworth, Etheridge, jun., and Nicholson,’ from the 
Whitehouse Group of the Girvan district. The rest of the genal 
surface of 7. Bureau is said to be smooth or minutely punctate, 
and no reticulating lines are described. Ocular tubercles are 
absent. 
2. We pass now to consider the typical members of the second 
group in which a definite ocular ridge and ocular tubercle is developed 
in each genal area. These structures find their expression in most 
of the members of McCoy’s genus Zretaspis. The ocular ridge starts 
at the level of and opposite the first lateral furrow of the glabella, 
and thus corresponds in place of origin with the marginal tubercle 
and commencement of the radiating lines in Z. Murchisoni and its. 
allies. Usually the ridge ends in a small tubercle called the ocellus 
or ocular tubercle by most authors, and it may run out at right angles 
to the axial furrows or be directed rather obliquely backwards 
towards the postero-lateral angle. It is occasionally continued 
beyond the ocular tubercle to this point with diminished strength, as 
may be seen in the smaller specimens from Thraive Glen, Girvan, 
which are attributed to 7. Bucklandi, Barr. In these this extension 
of the ridge ends against the fringe a little in front of the postero- 
lateral angle of the genal area. Nicholson & Etheridge, jun.,” noticed 
and described this feature in this Scotch species, and the present 
author * has also described it in 7. subradiatus from the same area. 
Ruedemann‘ has observed its occurrence also in JZ. reticulatus, 
Ruedemann. The inner and outer portions of the ocular ridge do 
not correspond in position or course with the line of bending which 
crosses the genal areas in the first group; but as above pointed out 
the point of origin of the ocular ridge coincides with that of the 
group of nervures. © 
Not only may the outer portion or extension of the ocular ridge 
disappear, as is the case in the large form of 7. Bucklandi from 
Girvan, but the inner portion between the axial furrow and ocular 
tubercle may likewise become obsolete, leaving the latter isolated. 
Since the young of 7. concentricus (Eaton) possesses an ocular ridge 
(but not its extension) as well as an ocular tubercle, according to 
Beecher,*® both structures have been held to be phylogenetically 
primitive characters; and if this is sothe loss of the outer and inner 
1 Reed, Girvan Trilobites (Paleont. Soc.); pt. i, p. 25, pl. iv, fig. i, 1903. 
* Nicholson & Etheridge, Mon. Silur. Foss. Girvan, fase. 2, p. 191, 1880. 
3 Reed, Girvan Trilobites, pt. i, p. 12, pl. ii, figs. 1-6, 1903. 
4 Ruedemann, Bull. 49 New York State Museum (1901), Paleont. Papers 2, 
p. 41, pl. iii, figs. 11, 15-20. 
° Beecher, Amer. Journ. Sci., vol. xlix, p. 309, pl. iii, 1895. 
