172 Ff. kh. C, Reed—On the genus Trinucleus. 
trilobites nearer the annelidan stock and with less fixed ordinal 
characters might have more segments bent round ventrally than 
higher forms, and the degree of coalescence of these inverted segments 
might vary. Thus, in the protaspis of Zrvarthrus there are two 
segments in front of the glabella which does not reach the anterior 
margin as it does in Sao; and ifthe immediate ancestors of 7rinucleus 
had such additional anterior somites one of them might have given 
rise to the ‘‘ lower plate”’, which, with its genal spines, would thus 
constitute an exceptionally modified and unfused segment. The 
condition of the adult head-shield of Zrinucleus would thus have | 
retained more primitive features than any other trilobite. We must 
admit that the ontogeny of 7. concentricus, so far as it is known, does 
not support this theory, but we must remember that the larval stages 
of the earliest species of Zrinucleus are unknown, and we have been 
led from previous considerations to regard Z. concentricus as one of the 
more specialized forms. In some other trilobites also the somite repre- 
sented by the free cheeks (the so-called ‘‘ ocular segment” of Walcott) 
might not be the first one in the composition of the cephalic shield. 
The rostral suture of Calymene and other genera on the above 
theory might be regarded as the abbreviated representative of the 
same suture as the marginal one in Zrinucleus, and the epistome as 
corresponding to the lower plate of the fringe. 
Walcott,’ by his study of the Mesonacide, has been led to consider 
it as not improbable that a seventh segment more anterior than the 
‘ocular segment” (i.e. the segment comprising the free cheeks and 
carrying the compound eyes) existed in the primitive cephalon of the 
Mesonacide, and he sees traces of it in the larval structure of Olenellus 
Gulbertt and in the cephalon of Olenelloides and Callavia bicensis; so 
that in tabulating the somites included in the cephalon he gives first 
the ‘‘ anterior border segment’, secondly the ‘‘ ocular segment”’, and 
thirdly the first glabellar segment from which the ocular ridge and 
palpebral lobe are developed. 
It may here be mentioned that though the genal spines belong 
to the ‘‘ocular segment’ in the Opisthoparia, yet in the Proparia 
they belong to one of the posterior segments and perhaps to the 
fourth glabellar segment of the Mesonacide, which in some genera 
(e.g. Olenelloides) is produced at its lateral termination to form the 
so-called intergenal spines on the posterior border. The genal spines 
of the Proparia are therefore not homologous structures with those 
of the Opisthoparia, a point of importance rarely recognized ; and it 
would therefore not be wholly contrary to expectation to find the 
genal spines of the genus Zrinucleus belonging to a different somite 
of the cephalon, and possibly to the first or ‘‘ anterior border” one of 
Walcott, bent under ventrally. 
Whatever value may be attached to the above theoretical con- 
siderations, it must be admitted that there is a possible interpretation 
on these lines, though it would upset our whole accepted ideas of the 
morphological correlation of the parts of the head-shield. 
Another suggestion which may be offered to explain the mar reinal 
suture is that it is connected with the process of ecdysis; for Limulus 
1 Walcott, Smithsonian Mise. Coll., vol. liii, No. 6, p. 238, 1910. 
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