NATURAL CLASSIFICATION OF THE TRILOBITES 127 



segment is frequently marked by two entirely separate lateral 

 lobes, as in Acidaspis, Oonolichas, Chasmops, etc. Likewise, 

 the fourth annulation carrying the first pair of maxillae is 

 often similarly modified in the same genera, also in all the 

 Proetidse, and in Cheirurus, Crotalocephalus, Sphoerexochus, 

 Ampyx, Harpes, etc. Here again, among adult forms, the 

 stages of progressive differentiation may be taken as indi- 

 cating the relative rank of the genera. 



The comparative areal growth of the free-cheeks is ex- 

 pressed by the gradual moving of the facial suture toward 

 the axis. As the free-cheeks become larger the fixed-cheeks 

 become smaller. In the most primitive protaspis stages and 

 in Agnostics, Harpes, and Trinucleus the dorsal surface of the 

 cephalon is wholly occupied by the axis and the fixed-cheeks, 

 while in the higher genera the area of the fixed-cheeks 

 becomes reduced until, as in Stygina and Phillipsia, they 

 form a mere border to the glabella. Therefore the ratio 

 between the fixed- and free-cheeks furnishes another means 

 of assisting in the determination^ of rank. 



The pleura from the segments of the glabella are occasion- 

 ally visible, as in the young of Elliptocephala, but usually 

 the pleura of the neck segments are the first and only ones to 

 be distinguished on the cephalon, the others being so com- 

 pletely coalesced as to lose all traces of their individuality. 

 The pleura of the pygidium appear soon after the earliest 

 protaspis stage, and in some genera (Sao, Dalmanites) are 

 even more strongly marked than in the adult state and much 

 resemble separate segments. The growth of the pygidium is 

 very considerable through the protaspis stage. At first it 

 is less than one-third the length of the dorsal shield, but by 

 successive addition of segments it soon becomes nearly one- 

 half as long. In some genera it is completed before the 

 appearance of the free thoracic segments, all of which are 

 added during the nepionic stages. An interpretation of 

 these facts to apply in valuing adult characters would indi- 

 cate that a very few segments, both in thorax and pygidium, 

 may be evidence of arrested development or degeneration. 



