SUBCLASS I 



TRILOBITA 



705 



shield is practically simultarieous with these organs, and before the eyes have 

 travelled over the margin, the free cheeks must be wholly ventral in position. 

 When first discernible, they are very narrow, and in Ptychoparia and Sao, 

 include the genal angles. In Dalmanites and Cheirurus, however, the genal 

 angles are borne on the fixed cheeks. 



Since the free cheeks are ventral in the earliest larval stages of all but the 

 highest Trilobites, and as this is an adult feature among a number of genera, 

 which on other grounds are very primitive, this is taken as generally indicative 

 of a very low rank. The genera Harpes, Agnostus, Cryptolithus and their allies 

 agree in this respect, and constitute the Hypoparia. 



The remaining genera of Trilobites present two distinct types of head 

 structure, dependent upon the extent and character of the free cheeks. In 

 both, the free cheeks make up an essential part of the dorsal crust of the 

 cephalon, being continued on the ventral side only as a doublure or infolding 

 of the edge, similar to that of the free edge of the cranidium, the ends of the 

 thoracic pleura, and the margin of the pygidium. They may be separated 

 only by the cranidium, as in Ptychoparia; by the cranidium and separate 



AB CD 



Fig. 1355. 



Ontogeny of Sao Mrsuta Barr. (Opisthoparia) A, Protaspis. B, Cephalon of nepionic individual. C, Cephalon of 

 later nepionic individual having eight free segments. D, Cephalon of adult (froni Beecher, after Barrande). 



epistomal and rostral plate, as in Illaenus and Homalonotus ; or they may be 

 united and continuous in front, as in Aeglina and Dalmanites. One type of 

 structure is distinguished by having the free cheeks include the genal angles, 

 thus cutting ofF more or less of the pleura of the occipital segment. The 

 genera belonging to this group constitute the second order — the Opisthoparia. 



The third and last type of structure includes forms in which the pleura of the 

 occipital segment extend the füll width of the base of the cephalon, embracing 

 the genal angles. The free cheeks are therefore separated from the cranidium 

 by sutures cutting the lateral margins of the cephalon in front of the genal 

 angles. Genera having this structure are here placed in the order Proparia. 



The characters still to be noticed have chiefly family and generic values, 



Fig. 135G. 



Onlogeny of Dalmanitina socialis (Barr.). (Proparia) A, Protaspis. B, Cephalon of individual of three free 

 Segments. C, Cephalon of one with seven free segments. D, Cephalon of adult (from Beecher, after Barrande). 



and are of great assistance both in determining the place of a family in an 

 Order and the rank and genetic position of a genus in a family. 



There is very satisfactory evidence that the eyes have migrated from the 

 ventral side, first forward toward the margin, and then backward over the 

 cephalon to their adult position. The most primitive larvae should therefore 

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