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 



FIG. 12V9. 



Ontogeny of Sao hirsuta, Barr. (Opisthoparia) A, Protaspis. 7J, 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 



FIG. 1280. 



Ontogeny of Dalmanites sodalis, Barr. (Proparia) A, Protaspis. B, Cephalon of individual of three free 

 segments. C, Cephalon of one of seven free segments. 1), Cephalon of adult (from Beecher, after Barrande). 



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 Ptyclioparia, 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 Triar- 

 thrus. Sao retains the eye-line throughout life, but in Triarthrus 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 disappears from the adult stages (Triarthrus); 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 

 Proetus, Acidaspis, Arges, and Dalmanites, through acceleration, they are present 

 in all the protaspis stages, and persist to the mature or ephebic condition, 



