F. R. C. Reed—On the genus Trinucleus. ATS 
no genal tubercles, though in some cases it appears to be completely 
absent. 
5. Specialization. 
It is not only in the head-shield of Zrinucleus that we can observe 
extraordinary specialization and the modification or loss of phylo- 
genetically primitive characters. For the small and constant number 
of the thoracic segments, and the uniform type of pygidium as well 
as its relatively large size and its composition of many fused 
segments, indicate a considerable divergence from and advance beyond 
the condition of the many-ringed thorax and small pygidium of the 
Conocoryphide and Olenide or even the Harpedide, in spite of the 
latter having often been regarded as closely allied to Zrinucleus. 
It does not seem that Zrinucleus is in the direct line of any of the 
other groups of trilobites, but was an early offshoot of the Opistho- 
paria, to which it is linked (though not by direct phylogenetic 
connection) by Orometopus and especially Ampyx. Its nearest 
homcomorphs are Dionide and Harpes, but the latter retains more 
of the primitive trilobite characters (e.g. many segments to the 
thorax and a small pygidium), and does not show such extreme 
specialization except in the head-shield. 
Instead, therefore, of placing Zrinucleus as one of the simple lowly 
types of trilobites illustrating an early stage in the history of the 
class, it seems more probable that we should regard it as a modified 
and degenerate form belonging to the Opisthoparia which has been 
specialized and adapted to a peculiar environment. 
Summing up the results of our study of the head-shield of Zrinucleus 
and balancing the probabilities of the various theories in connection 
with its structure and origin, we seem led to conclude that the 
ancestors of this genus branched off from an Opisthoparian stock and 
suffered degeneration of certain parts in combination with extreme 
specialization of other parts to fit them for a peculiar environment. 
Under the aphotic condition of the benthos burrowing in the mud 
rendered compound eyes unnecessary and they therefore degenerated, 
while in compensation for the loss of these organs a complete 
system of nervures developed over the glabella and cheeks. The 
fixed and free cheeks fused along the line of the facial sutures and 
ultimately became completely obliterated. The border of the head- 
shield broadened out and was folded in ventrally so as toform a lower 
plate, while to facilitate moulting a line of fission was formed along 
its edge. Perforation of both layers of this border took place on an 
extensive scale, and the corresponding pits of the two layers 
frequently communicated. A change of habits in some of the 
species led to the development of ocelli by rudiments of the compound 
eyes persisting from larval to adult life, but in other cases this 
renewal of visual organs was found of no value, and therefore again 
lost. In conjunction with the presence of the ocelli the optic nerve 
(ocular ridge) was strengthened and redefined. These changes, which 
suggest the retention till maturity of larval features or the 
re-acquisition of organs of which the rudiments had only been left, 
do not appear to have been strictly successional, but to have been 
_ developed as occasion required. 
