SUB-CLASS I TRILOBITA 621 
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 application 
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) submarginal 
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 7rarthrus 
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 
smooth glabella, as in L/laenus 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, Cono- 
lichas, Chasmops, ete. Likewise, the fourth annulation carrying the first pair 
of maxillae is often similarly modified in the same genera, also in all the 
Proétidae, and in Cheirurus, Crotalocephalus, Sphaerexochus, Ampyx, Harpes, ete. 
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 T'rinucleus, 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 
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 Liliptocephala, 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 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 ot 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 
