620 ARTHROPODA SUB-KINGDOM VII 
structure is distinguished by having the free cheeks include the genal angles, 
thus cutting off more or less of the pleura of the occipital segment. The 
genera belonging to this group constitute the second order—the Opisthoparia. 
The third and last type of structure includes forms in which the pleura of 
the occipital segment extend the full width of the base of the cephalon, em- 
bracing the genal angles. The free cheeks are therefore separated from the 
cy AN 
. 1279. 
A 

Ontogeny of Sao hirsuta, Barr. (Opisthoparia) A, rae B, Cephalon of nepionic individual. C, Cephalon 
of later nepionic individual having eight free segments. D, Cephalon of adult (from Beecher, after Barrande). 
cranidium by sutures cutting the lateral margins of the cephalon in front of 
the genal angles. Genera having this structure are here placed in the order 
Proparia. 
The characters still to be noticed have chiefly family and generic values, 
and are of great assistance both in determining the place of a family in an 
order and the rank and genetic position of a genus in a family. 
There is very satisfactory evidence that the eyes have migrated from the 
ventral side, first forward toward the margin, and then backward over the 
py AD 
. 1280. 
Ontogeny of Dalmanites socialis, Barr. (P ab A, Protaspis. B, Cephalon of individual of three free 
segments. C, Cephalon of one of seven free segments. D, Cephalon of adult (from Beecher, after Barrande). 
A 

cephalon to their adult position. The-most primitive larvae should therefore 
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 T'riar- 
thrus. Sao retains the eye-line throughout life, but in 7riarthrus 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 dis: appears from the adult stages (Z’riarthrus) ; 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 
Provtus, Acidaspis, Arges, and Dalmanites, through acceleration, they are present 
in all the protaspis stages, and persist to the mature or ephebic condition, 
~~ h-— 
