706 ARTHROPODA phylum vii 



present no evidence of eyes on the dorsal shield. Just such conditions are 

 fulfilled in the youngest larvae of Ptychoparia, Solenopleura and Liostracus. 

 The eye-line is present in the later larval and adolescent stages of these genera, 

 and persists to the adult condition. In Sao it has been pushed forward to the 

 earliest protaspis, and is also found in the two known larval stages of Triar- 

 thrus. Sao retains the eye-line throughout life, but in Triarthnis the adult has 

 no trace of it. A study of the genera of Trilobites shows that this is a very 

 archaic feature, chiefly characteristic of Cambrian genera, and only appearing 

 in the primitive genera of higher and later groups. It first develops in the 

 later larval stages of certain genera (Ptychoparia, etc.) ; next in the early larval 

 stages (Sao) ; then disappears from the adult stages {Triarthnis) ; and finally 

 is pushed out of the ontogeny (Dalmanites). 



In Ptychoparia, Solenopleura, Liostracus, Sao and Triarthrus, the eyes are 

 first visible on the margin of the dorsal shield after the protaspis stages have 

 been passed through, and later than the appearance of the eye-lines ; but in 

 Pro'etus, Acidaspis, Ceratarges and Dalmanites, through acceleration, they are 

 present in all the protaspis stages, and persist to the mature or ephebic condi- 

 tion, moving in from the margin to near the sides of the glabella. Progression 

 in these characters may be expressed, and in so far taken for general applica- 

 tion among adult forms to indicate rank, as follows: — (1) Absence of eyes; 

 (2) eye-lines ; (3) eye-lines and marginal eyes ; (4) marginal eyes ; (5) sub- 

 marginal eyes ; (6) eyes near the pleura of the neck segment. 



The changes in the glabella are equally important and interesting. 

 Throughout the larval stages the axis of the cranidium shows distinctly by 

 the annulations that it is composed of five fused segments, indicating the 

 presence of as many paired appendages on the ventral side. In its simplest 

 and most primitive state it expands in front, joining and forming the anterior 

 margin of the head (larval Ptychoparia and Sao). During later growth it 

 becomes rounded in front, and terminates within the margin. In higher 

 genera, through acceleration, it is rounded and well defined in front, even in 

 the earliest larval stages, and often ends within the margin (larval Triarthrus 

 and Acidaspis). From these few simple types of pentamerous glabellae, all 

 the diverse forms among species of various genera have been derived, through 

 changes affecting any or all of the lobes. The modifications usually consist in 

 the progressive obsolescence of the anterior annulations, finally producing a 

 smooth glabella, as in Illaenus and Niohe. The neck segment is the most per- 

 sistent of all, and is rarely obscured. The third or mandibular segment is 

 frequently marked by two entirely separate lateral lobes, as in Acidaspis, Cono- 

 lichas, Chasmops, etc. Likewise, the fourth annulation carrying the first pair 

 of maxillae is often similarly modified in the same genera, also in all the 

 Proetidae, and in Cheirurus, Crotalocephalus, Sphaevexochus, Ampyx, Harpes, etc. 

 Here, again, among adult forms, the stages of progressive differentiation may 

 be taken as indicating the relative rank of the genera. 



The comparative areal growth of the free cheeks is expressed 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 pro- 

 taspis stages, and in Agnostus, Harpes and Cryptolithus, the dorsal surface of the 

 cephalon is wholly occupied by the axis and 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 



