NATURAL CLASSIFICATION OF THE TIULOBITES 119 



head, and on the basis that each neuromere corresponds to 

 an original segment, as on the post-cephalic region, there 

 would need to be this number accounted for. The anterior 

 segment, or number one in the trilobites, would be repre- 

 sented by the hypostoma ; the second segment, by the paired 

 eyes, free-cheeks, and rostral plate ; the third, by the anterior 

 lobe of the glabella and the first antennae ; the fourth, by the 

 second lobe of the glabella and the second pair of antennoe ; 

 the fifth, by the third lobe of the glabella and the mandibles ; 

 the sixth, by the fourth lobe of the glabella and the first 

 maxillte; and the seventh, by the neck lobe, or occipital 

 ring, and the second pair of maxillse. The five annulations, 

 or lobes, of the axis of the cranidium, since they primarily 

 carry fulcra for the attachment of muscles supporting or 

 moving the appendages, could thus be interpreted in tetms of 

 the ventral structure, making the first lobe the antennulary, 

 the second the antennary, the third the mandibular, the 

 fourth the first maxillary, and the fifth the second maxillary. 

 No other group of Crustacea furnishes such constant and 

 well-developed structures representing the second theoretical 

 head segment, which is obscure or obsolete in all the living 

 groups, excepting probably the stalked eyes of some Crus- 

 tacea and the movable ocular segment of the Stomatopoda. 

 For this reason, in addition to the many other important 

 differences previously noted in tlie table of comparative 

 morphology, the trilobites are regarded as a sub-class, and 

 the relative denomination and structural relations of this 

 second segment, along with other characters, are considered 

 as of sufficient physiological and morphological importance to 

 determine the ordinal divisions. 



Principles of a Natural Classification. 



Most satisfactory and conclusive results have already come 

 from the application of the law of morphogenesis, or the 

 recapitulation theor}', to various groups of animals, by means 

 of which their natural classification and phylogenetic rela- 

 tions have been determined. Hyatt ^^ says on this point 



