F. R. C. Reed—On the genus Trinucleus. 173 
and Apus are said to moult by splitting along the frontal edge of the . 
carapace. This would then have to be regarded as a special adaptation, 
and perhaps of no phylogenetic importance. The facial sutures in 
trilobites are usually considered to have been of some use in ecdysis, 
and compensation for their loss in Zrinucleus may have been obtained 
by this means. 
The fringe would according to this hypothesis be merely brought 
about by the flattening and broadening of the ordinary cephalic 
border, while the perforations through its substance might be derived 
from the multiplication of pits similar to those seen in the marginal 
furrow of Dionide,’ Harpes, and Euloma.* The lower plate of the 
fringe would represent the doublure or reflexed portion of the border. 
The complete fusion of the pits of the upper and lower surfaces of 
the fringe would be secondary. 
3. Ocular Ridges and Nervures. 
The history and relations of the ‘‘ ocular ridge” or ‘‘eye-line’”’ in 
Trinucleus now demand attention. It is generally looked upon as 
a primitive character. Walcott (op. cit.) has shown how it forms 
part of the third cephalic (i.e. first glabellar) segment in the case 
of the Mesonacide; and in Cambrian trilobites belonging to the 
- Opisthoparia as well as to the Harpedide this structure is well . 
developed, while, as is well known, it persists in some later genera. 
It is usually accepted without question that the similarly named 
structure in Zrinucleus is homologous with this ‘‘eye-line”’ of the 
Olenide, ete., though in the latter its point of origin does not appear 
to be so constant, for it frequently arises close to the anterior end of 
the glabella instead of strictly opposite the first glabellar furrow. 
It also runs out to compound eyes, not to ocelli, and the eyes are 
situated on the facial sutures. It is remarkable that in the earliest 
representatives of the genus Zrimucleus (Group 1) the typical ocular 
ridge does not occur; but it is found in the larval stages of Z. con- 
centricus, and is well developed in the adult of 7. setecornis of our 
second group. Till we know the ontogeny of 7. Murchisoni or its 
allies we cannot come to the conclusion that the irregular radiating 
bunch of nervures is more primitive than the definite ocular ridge 
or optic nerve, at any rate in this genus. But the stratigraphical 
succession of species points this way, if we dismiss the idea of 
degeneration. 
In most of the Conocoryphide® (e.g. C. Sulzert, Schloth., and 
Erinnys venulosa, Salt.) there is one of the nervures more strongly 
developed than the rest, and it forms the trunk from which they 
branch; its position corresponds with that of the eye-line of the 
Olenide. If we regard the ocelli of Zrinucleus of Group 2 to be the 
degenerate successors of the compound eyes of other trilobites and 
believe that the facial sutures have been obliterated by the complete 
fusion of the free and fixed cheeks, there is no difficulty in accepting 
the view supported by Lake and Swinnerton (op. cit.) that the ocular 
' Reed, GEOL. MaG., Dec. V, Vol. IX, p. 200, Pl. XI, Figs. 3-6, 1912. 
* Brégger, Die Silur. Htagen 2 und 3 (1882), p. 98; Reed, GEOL. MaG., 
Dec. IV, Vol. VII (1900), pp. 251, 255. 
® Grénwall, Bornholms Paradoxideslag, pp. 82-104, 1902. 
