SUB-CLASS I TRILOBITA 619 
as indicated by the additional grooves on the axis and limb. In the earliest, 
or Cambrian genera, the protaspis stage is by far the simplest expression of 
this period to be found. In the higher and later genera, the process of 
acceleration or earlier inheritance has pushed forward certain characters until 
they appear in the protaspis, thus making it more and more complex. 
Taking the early protaspis stages in Solenopleura, Liostracus, or Ptychoparia, 
it is found that they agree exactly with the foregoing diagnosis in its most 
elementary sense. Since they are the characters shared in common by all 
larvae at this stage, they are taken as primitive, and accorded that value in 
dealing with adult forms possessing homologous features. Therefore, any 
Trilobite with a large elongate cephalon, eyes rudimentary or absent, free 
cheeks ventral or marginal, and glabella 
long, cylindrical, and with five annulations, 
would naturally be placed near the beginning 
of any genetic series, or as belonging to a (Uy) 
4 
A B C 

very primitive stock. 
Next must be considered the progressive 
addition of characters during the geological Ee 
history of the protaspis, and the ontogeny 
of the individual during its growth from 
the larval to its mature condition. It has 
been shown by Beecher that there is an 
exact correlation to be made between the 

geological and zoological succession of first 
larval stages and adult forms, and therefore Fic. 1278. 
both may be reviewed together. Sao hirsuta, Barr. Cambrian; Skrey, 
: Bohemia. A, Protaspis. B-F, Nepionic stages 
The first important structures not of development (after Barrande). 
especially noticeable in all stages of the 
protaspis are the free cheeks, which usually manifest themselves in the meta- 
or para-protaspis stages, though sometimes even later. Since they bear the 
visual areas of the eyes, when such are present, their appearance on the dorsal 
shield is practically simultaneous with these organs, and before the eyes have 
travelled over the margin, the free cheeks must be wholly ventral in position. 
When first discernible, they are very narrow, and in “Ptychoparia and Sao, 
include the genal angles. In Dalmanites and Cheirurus, however, the genal 
angles are borne on the fixed cheeks. 
Since the free cheeks are ventral in the earliest larval stages of all but the 
highest Trilobites, and as this is an adult feature among a number of genera, 
which on other grounds are very primitive, this is taken as generally indicative 
of a very low rank. The genera Huarpes, Agnostus, Trinucleus, and their allies 
agree in this respect, and constitute the Hypoparia. 
The remaining genera of Trilobites present two distinct types of head 
structure, dependent upon the extent and character of the free cheeks. In 
both, the free cheeks make up an essential part of the dorsal crust of the 
cephalon, being continued on the ventral side only as a doublure or infolding 
of the edge, similar to that of the free edge of the cranidium, the ends of the 
thoracic pleura, and the margin of the “pygidium. They may be separated 
only by the cranidium, as in Ptychoparia,; by the cranidium and_ separate 
epistomal or rostral plate, as in J/lacnus and Homalonotus ; or they may be 
united and continuous in front, as in Aeglina and Dalmanites. One type ot 
