NATURAL CLASSIFICATION OF THE TRILOBITES 113 



ment of the pygidium for dividing the trilobites into groups 

 larger than families, and it seems evident from the present 

 state of knowledge that it is impossible to make this charac- 

 ter of more than family or even generic value. Many of the 

 genera which must naturally be included in the Archiaspides 

 have pygidia that cannot be said to be rudimentary, obsolete, 

 or wanting in function. Even those genera having pygidia 

 with few segments, as Mesonacis, Holmia, Paradoxides, Sele- 

 nopeltis, Dicranurus, Bronteus, Harpes, etc., show in many 

 other more important characters that they are highly differ- 

 entiated arid specialized forms, and that this feature is one 

 expression of such development. The futility of the scheme 

 is at once evident when a comparison is made between allied 

 genera which present marked differences in the size and 

 segmentation of the pygidium; as Phacops and Dalmanites, 

 Ceraurus and Encrinurus, Calymmene and Homalonotus, Harpes 

 and Trinucleus, Mesonacis and Zacanthoides, Paradoxides and 

 Dikelocephalus. 



The last classification to be noticed is that of E. J. Chap- 

 man, 13 in 1889, in which four sub-orders or primary groups are 

 proposed, differing considerably from any previous arrange- 

 ment, and based upon arbitrary features of general structure 

 and configuration, especially the form of the glabella, whether 

 wide, conical, or enlarged. Twenty-seven families are rec- 

 ognized. In this scheme Trinucleus, Ampyx, and ^glina 

 form one section ; Paradoxides and Acidaspis, together with 

 Phacops and Encrinurus, another; all under one sub-order. 

 Omitting the Agnostidae, there are here considered in a single 

 sub-order the most characteristic representatives of nearly all 

 the types of trilobite structure. Proetus, Cyphaspis, and 

 Arethusina fall into three sections, under two sub-orders, 

 although these genera, on account of their great similarity 

 in essential points, are placed in a single family by most 

 authors. A further analysis of this classification in its 

 broader lines would be unprofitable. It is sufficient to state 

 that the facts obtained from the study of the ontogeny of 



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