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



lines in the two orders (Figs. \b, d). In Opisthoparia the suture 

 becomes visible for the first time at the postero-lateral margin of the 

 head-shield ; in Proparia, on the contrary, it appears at the antero- 

 lateral margin. This difference is exhibited in adult life also by the 

 lowliest members of the two orders ; compare, for example, Conocoryphe 

 and Anacheirurns (Figs, la, c). The opisthoparian and proparian 

 conditions are therefore differentially or mutually exclusive and have 

 arisen independently. As will be shown later, this conclusion receives 

 confirmation from the existence of evidence that the opisthoparian 

 condition has been evolved independently at least twice within the 

 limits of the order Opisthoparia. 



The Calymmenid^ and Hohalonotidje. 



As long ago as 1898 Pompeckj 1 traced the descent of the 

 Calymmenidse through Pharostoma to Bavarilla; Pharostoma has 

 narrow marginal cheeks of the opisthoparian type and Bavarilla is 

 one of the Olenidse. 2 He likewise traced the Homalonotidse back to 

 the olenid genus Neseuretus. In other Olenidse, 2 e.g. Triarthrus, the 

 facial suture cuts the margin at the genal angle. It is evident, 

 therefore, that among some of the olenid Opisthoparia one tendency 

 of specialization is the shifting of the post-ocular facial suture towards 

 the genal angle. This tendency develops most rapidly in, and 

 becomes the dominant peculiarity of, the Calymmenidse and the 

 Homalonotidse. 



Gurich grasped the importance of this peculiarity and formed 

 a group, the Gonatoparia, to include those families which show it. 

 This group has not the same taxonomic value as the two orders ; it 

 might, however, be regarded as a section, the Calymmenina, of that 

 large and unwieldy assemblage the Opisthoparia. 



The ereok in Gurich's Classification. 



A glance at a system of classification should reveal first the main 

 lines of descent and after that the progressive stages along each 

 of those lines. For defining lines of descent differential characters 

 are the most reliable, hence the solidity of two of Beecher's orders. 

 The foundation mistake made by Gurich is that his main divisions are 

 based upon progressive characters, viz. differences in the number 

 of free segments and in the size of the pygidium. Beecher's order 

 Hypoparia exhibits the same weakness. The institution of an order 

 Protoparia is open to the same criticism. 



It is beyond question that the earliest ancestors of the Trilobites and 

 indeed of all Arthropods possessed an annelidan type of segmentation.* 

 In all Arthropods this has been masked in the anterior part of the 

 body by the fusion of segments. This process of cephalization 

 reaches its acme in the crab-like Crustacea. It is characteristic of 

 the Trilobite organization that it also exhibits a strong tendency for 

 a similar fusion to take place at the posterior end of the body. 

 Gurich emphasizes the importance of this tendency by unconsciously 



1 Neues Jahrb., 1898, p. 187. 



2 Strictly that portion of Olenidse relegated later to the Ptychoparidffl. 



3 Cf. H. M. Bernard, Q.J.G.S., 1894, 1895. 



