SUB-CLAss I 



moving in from the margin ID near the sides .t tin- glaKella. Progression in 

 these characters may lie expressed, and in so far taken for general application 

 among adult forms to indicate rank, as follows : (1) Absence of eyes ; (2) ey^i- 

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

 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 ly 

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

 presence of as manv 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 Ptyckoparia and N</</). During later growth it 

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

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

 the earliest larval stages, and often ends within the margin (larval Tim rtlirn* 

 and Acidaspit). 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 Niobe. 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, Oono- 

 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, Sphaerexochus, 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 Triuucleus, 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 &///////// 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 occasionally visible, as 

 in the young of JElliptocephala, but usually the pleura of the neck segments are 

 the first and only ones to be distinguished on the cephalon, the others being 

 so completely coalesced as to lose all traces of their individuality. The pleura 

 of the pygidium appear soon after the earliest pro taspis 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 i.- 

 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 indicate that a very few segments, both in thorax and 

 pygidium, may be evidence of arrested development or suppression. On the 

 other hand, the apparently unlimited multiplication of thoracic and especially 



