124 STUDIES IN EVOLUTION 



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. It seems to mean that the second segment 

 remains where it was mechanically placed, and retains its 

 full somitic nature, though from the necessities of such a 

 condition true ventral segments must soon disappear through 

 modification into other structures or through disuse as seg- 

 ments. The genera Harpes, Agnostus, Trinucleus, and their 

 allies agree in having well-developed, continuous, ventral 

 free-cheeks, and constitute a natural group. As they pos- 

 sess one expression or type of the genesis of an important 

 common character, based upon facts of development, it should 

 stand as an ordinal character, and as such it is here taken. 

 For this group the name Hypopaeia is proposed. It is fully 

 defined and its limitations established in the proper place in 

 the classification. 



The remaining genera of trilobites present two distinct 

 types of head structure, dependent upon the extent and char- 

 acter 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 Ptychopnria^ or by the cranidium and rostral plate, as in 

 Illcenus and Homalonotus, or they may be united and con- 

 tinuous in front, as in ^^glina 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 full 

 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 



