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been seen in a sufficiently representative series of genera to make it rea- 
sonably certain that it is the common larval type among the trilobites. 
It is pretty well established that the eye of crustaceans has migrated 
from the ventral to the dorsal surface of the cephalon. At an interme- 
diate stage in this process the eyes would appear on the margins of the 
cephalon. If this has been the history of the eye, the most primitive 
larvae should show no evidence of eyes on the dorsal surface, and since 
the eye is on the inner margin of the free cheek, there should be no evi- 
dence of the free cheek. This is exactly the case in the youngest larvae 
ot Ptychoparia, Solenopleura and Liostracus, “which are the most primi- 
tive genera whose protaspis is known. The eye-line is present in the 
later jarval and adolescent stages of these genera, and persists to the 
adult condition. In Sao it has been pushed forward to the earliest protas- 
pis, and is also found in the two known larval stages of Triarthrus. Sao 
retains the eye-line throughout life, but in Vriarthrus the adult has no 
traces of it, and none of the higher and later genera studied has an eye- 
line at any stage of development.” This character according to Matthews, 
is characteristic of the Cambrian trilobites. In its phylogenesis in later 
trilobites it disappears first frem the adult stages, and is finally lost 
from the entire ontogeny. The eyes appear on the margin of the cephalon 
in the last larval stage of Ptychoparia, Solenopleura, Liostracus, Sao, and 
Triarthrus. In the later genera the eyes 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.”’ 
According to Beecher (8S) “A number of genera present adult char- 
acters which agree closely with some of the larval features [of later 
genera]. The main features of the cephalon in the simple protaspis forms 
of Solenopleura, Liostracus, and Ptychoparia are retained to maturity in 
such genera as Carausia and Acontheus, which have the glabella expanded 
in front, joining and forming the anterior margin. They are also without 
eyes or eye-line. Ctenocephalus vetains the archaic glabella to maturity, 
and likewise shows eye-lines and the beginnings of the free cheeks (larval 
Sao). Conocoryphe and Ptychoparia are still further advanced in having 
the glabella rounded in front, and terminated within the margin (larva of 
Triarthrus). 'These facts and others of a similar nature show that there 
are characters appearing in the adults of later and higher genera, which 
successively make their appearance in the protaspis stage, sometimes to 
the exclusion or modification of structures present in the most primitive 
