120 F. R. C, Reed—On the genus Trinucleus. 
the smaller ones representing capillaries and venules as suggested 
by the genus Limulus. It can hardly be doubted that the general 
reticulation of the surface is not merely a superficial ornamentation 
of the shell of no functional importance, particularly when we are 
able to trace its direct connexion with a definite group of radiating 
vessels always arising from the same spot on the axial furrows. 
The small smooth or ciierenly, ornamented regions close to the 
base of the glabella suggest the ‘alar areas’ of Harpes and Dionide, 
which are also traceable in some species of Ampyax and are found 
clearly developed in larval stages of TZrinucleus as Beecher’ 
demonstrated. 
The nature and history of the semilunar outer band in front of the 
angulated line crossing the genal areas is intimately bound up with 
the whole interpretation of the structure of the head-shield of the 
genus. But before discussing this vexed question it will be best to 
describe the characters of the two other groups into which I have 
divided the species according to their genal features. 
It may, however, be naturally expected that in the earliest 
representatives of the genus which are comprised in my first group, 
and are confined to the Arenig and Llandeilo Beds, we should find 
the evidence of its affinities and original structure. Later species in 
the stratigraphical succession would be expected to diverge more — 
from the primitive stock and to have suffered additional modification. 
There is a link connecting the first group with the second, and it 
is found in the species Z. fimbriatus, Murchison, the position of which 
is intermediate. For it has no division of the genal areas into two 
parts by a line of angulation, their surface being gently and uniformly 
convex; there is no ocular tubercle and no typical ocular ridge, 
though Salter? and McCoy.* put this species in the genus Zretaspis, 
which is characterized by their possession. The diagonal facial 
suture which McCoy‘ clearly represented in his figure does not exist. 
There is, however, a small tubercle in young individuals® situated in - 
the axial furrow opposite the first lateral furrow of the glabella, just 
as in 7. Murchisont, but there is no such definite group of nervures 
radiating from it. over the genal area. We may, however, observe 
that near the posterior lateral angles of the genal areas the inter- 
lacing sinuous lines, which form the reticulations on the surface, 
converge and are grouped together to form larger, more elongated 
meshes directed towards a small tubercle near the posterior border 
with a pit in the pleuro-occipital furrow, asin 7. Murchisoni. The 
inner posterior angles are also nearly smooth with pees faint or 
obsolete reticulations, thus affording traces of the ‘alar areas’ 
The rest of the genal areas is covered with a polygonal or hexa- 
gonal fine meshwork. ; 
The French species 7. Bureaut, Oehlert,* which seems to belong 
1 Beecher, Amer. Journ. Sci., ser. 111, vol. xlix, pl. iii, figs. 1, 2, 1895. 
®? Salter, Dec. Geol. Surv., vii, pl. vii, fig. 8, 1853. 
* McCoy, Syn. Brit. Pal. Foss. Woodw. Mus., p. 146. 
* Ibid., pl. in, figs. 16, 16a. 
° Reed, GEOL. MAG., Dec. VI, Vol. I, Pl. XXVIII, Fig. 3, 1914. 
® Oehlert, Bull. Soc. géol. France, ser. WI, vol. xxiii, p. 308, pl. i, fig. 7, 
1895. 
