126 STUDIES IN EVOLUTION 



later groups or as an evidence of degeneration. It first 

 develops in the later larval stages of certain genera (PtycJio- 

 paria, etc.); next in the early larval stages (Sao)-, then dis- 

 appears from the adult stages (Triarthrus) ; and finally is 

 pushed out of the ontogeny (Dalmanites). 



In Ptychoparia, /Solenopleura, Liostracus, Sao, and Triar- 

 thrus 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 Proetus, 

 Acidaspis, Arges, and Dalmanites, through acceleration, they 

 are present in all the protaspis stages, and persist to the 

 mature or ephebic condition, moving in from the margin to 

 near the sides of the glabella. Progression in these char- 

 acters 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) 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 sim- 

 plest and most primitive state it expands in front, joining 

 and forming the anterior margin of the head (larval Ptycho- 

 paria 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 Triartlirm and Acidaspis). From these simple 

 types of simple pentamerous glabellsB all the diverse forms 

 among species of various genera have been derived, through 

 changes affecting any or all the lobes. The modifications 

 usually consist in the progressive obsolescence of the ante- 

 rior annulations, finally producing a smooth glabella, as in 

 Ulcenus and Niobe. The neck segment is the most persistent 

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



