329 



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 

 of Ptychoparia, Solenopleura and Mostracus, "which are the most primi- 

 tive genera whose protaspis is known. 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 protas- 

 pis, and is also found in the two known larval stages of Triarthrus. Sao 

 retains the eye-line throughout life, but in Triarthrus the adult has no 

 traces of it, arid 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 from 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 (8) "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 Aeon then s. which have the glabella expanded 

 in front, joining and forming the anterior margin. They are also without 

 eyes or eye-line. Ctenocephalus retains 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 



