544 Prof. H. H. Swinnerton — Classification of Trilobites. 



glabella. This combination of characters carries the Trinucleid line 

 of descent back to an early Conocoryphid-like stock which had not 

 yet lost the eyes, and in which caudalization had hardly begun. 

 The gap between Conocoryphe and known genera is, however, so 

 great that it is necessary to establish a separate sub -order, viz. the 

 Trinucleida. 



The Ellipsocephalidae fail to fulfil the conditions required of the 

 Trinucleid ancestral stock, because their first few segments have 

 already assumed an equality of size, and their remaining free 

 segments, together with the small pygidium, are already shortened 

 to such an extent as to produce an almost Trinucleid shape of body. 

 This similarity to the Trinucleida may be due to parallel adaptation. 

 On the other hand, it is not unreasonable to regard the Ellipsocephalidae 

 as an early offshoot of the same stock, and therefore subject to 

 the same morphological tendencies as those which characterize the 

 Trinucleida. 



Gurich places the Aeglinidae with the Trinucleidae. They certainly 

 exhibit the same type of trunk and tail region as that family and the 

 Ellipsocephalidae. They may be regarded as having the same 

 relationship to the Trinucleida that the Bemopleuridae have to the 

 Mesonacida or, in a less marked degree, Nileus to the Asaphidse, 

 viz. an adaptation to nocturnal pelagic or nektic mode of life. 1 



The Shumardiidae seem also to possess claims to at least a provisional 

 home among the Trinucleida inasmuch as nearly all their 

 characteristics in head-shield and body shape are paralleled among 

 the members of this sub-order. 



These three families — Ellipsocephalidae, Aeglinidae, and Shumar- 

 diidae — may be placed provisionally as an appendix to the Trinucleida. 



Sub-order Odontopleurida. 



Beecher, 2 speaking of the Odontopleuridae (Acidaspidae) and the 

 Lichadidae, says "these two families are closely related". The 

 tendency to develop numerous spines is an adaptation characteristic 

 of planktonie forms. The thinness of the carapace in the latter 

 family is also characteristic of pelagic animals. These features may 

 therefore be left out of account. The usual stability of the glabella 

 in other families and sub-orders emphasizes the genetic and classi- 

 ficatory value of the strong tendency towards the breaking up of the 

 glabella into separate lobes shown in these two families. Apart from 

 this the free cheek in the less modified forms is large and of the 

 Olenid type. There is at present an absence of connecting links 

 between these families and any of the other sub-orders. Until such 

 links are forthcoming a provisional sub-order, the Odontopleurida, 

 may be instituted. 



The Bronteidae (Goldiidae) present peculiar difficulty, for they are 

 a clearly defined family with no known connecting links with other 

 families. Beecher 3 points out the tendency in some species towards 

 •" a breaking up of the glabella into symmetrically disposed separate 



1 Cf . Dollo, vide supra. 



2 Op. cit., 1897, p. 197. 



3 Op. cit., p. 196. 



