TO THE EXISTING AIITICULATA. 43 



smaller division exhibits itself behind this one, and anterior to the articulation of tlie 

 succeeding one, we might thence infer that there were accessory parts of the mouth, an 

 inference which is highly probable, since in /Ipus there are also two pair of accessory parts 

 of the mouth at that spot. The structure of Limnadia and Branchipus, neither of which 

 possesses any true accessory parts of this kind, exhibits, however, an indication that these 

 may likewise have been wanting in those Trilobites characterized by a simple projecting 

 head. Such genera are Asaphis, Nileiis, Illanus, Trinudeus, 0(/i/(/ia, and Phacops (Division A), 

 all of which are Trilobites in which a pair of small bosses are always exhibited more or less 

 distinctly behind the large, simple projection of the head. These little bosses seem to 

 indicate the traces of the rings to which the jaws were attached, but I should be inclined to 

 question the existence of accessory parts of the mouth in all S'uch Trilobites ; and this 

 affords a decided reason for separating the species of Phacojjs referred to Division A, even 

 generically, a view which is also favoured by the whole structure of the body. 



It will now probably be admitted that in the structure of the PhyUopoda may be 

 recognized the typical characters exhibited in the general configuration and proportions of 

 the head in Trilobites, and that those writers have been fully justified who have considered 

 the two groups as related and nearly analogous. It will be seen as we advance, that 

 there are still additional reasons in support of the view I have taken, especially when we 

 come, in the next place, to consider the structure of the feet, since the varying numerical 

 proportion of the rings which compose the body of Trilobites separates the group from 

 PhyUopoda, where this number is constant (4 x 3 — 1). 



SECTION XXI. 



There is good proof that the feet of the Trilobites must have been soft membranous 

 organs, for the absence of the slightest remains of these organs in the numerous specimens 

 observed is of itself sufiicient evidence of the fact,* and it can indeed scarcely be supposed 

 that hard horny extremities should be affixed to a soft membranous abdominal surface ; 

 since they would not have then possessed that firm basis, which all solid organs of locomo- 

 tion require, in order that they may be properly available. That this abdominal surface also 

 must have been of a membranous nature seems quite clear, since it has in no instance been 

 preserved in a fossil state, whilst the hard, horny, perhaps calcareous, dorsal surface is 

 invariably retained, and there can be no reason why the latter only .should have been 

 handed down, if the former was also hard. We may then safely conclude that it was soft 

 and easily destroyed, and I would only have the reader refer to the rings in the tail of the 

 Crustacea, formed in a manner very similar to that observed in the case of the Trilobites, 

 in order that he may be convinced that if there had been hard and solid coverings of the 



* Eichwald (1. c. 39), Goklfuss, and Coimt Sternberg, as is well known, fancied they recognized 

 feet in the remains of some Trilobites ; but the representations and descriptions they have given are 

 too indefinite to enable us to draw any certain conclusion. Eichwald's description certainly mentions 

 the number of joints (five) and the size of the foot (foui- lines) with greater exactitude ; but even then 

 it is very unsatisfactory and obscure. 



