170 STUDIES IN EVOLUTION 



glabella with five annulations ; abdominal portion usually less 

 than one-third the whole length of the shield, axis with from 

 one to several annulations ; pleural portion smooth or grooved ; 

 eyes when present anterior, marginal, or sub-marginal ; free- 

 cheeks when present very narrow, marginal. 



Several moults took place during this stage before the 

 complete separation of the pygidium or the introduction of 

 thoracic segments. When such moults are recognized they 

 may be considered as early, middle, and late protaspis stages, 

 and designated respectively as anaprotaspis, metaprotaspis, 

 and paraprotaspis. They introduced various changes, such 

 as the stronger annulation of the axis, the beginning of the 

 free-cheeks, and the growth of the pygidial portion from the 

 introduction of new appendages and segments, as indicated by 

 additional grooves on the axis and pleura. Similar ecdyses 

 occur during the nauplius stage of many living Crustacea 

 before a decided transformation is brought about. Certain 

 of these later stages have received a distinctive name, and 

 are called the metanauplius. 



It is believed that the protaspis is homologous with the 

 nauplius or metanauplius of the higher Crustacea. Most of 

 the reasons for this belief will appear later in the present 

 paper ; some which may be stated now are as follows : — 



(1) The size of the protaspis does not differ greatly from 

 that of many nauplii, and represents as large an animal as 

 could be hatched from the bodies considered as the eggs of 

 trilobites. 



(2) Some of the sediments carefully examined by the 

 writer could preserve smaller larval trilobites were such 

 originally present and provided with a chitinous test, as 

 shown by the abundance of minute ostracodes, and the per- 

 fection of detail in these and other fossils. 



(3) The protaspis can be shown to be structurally closely 

 related to the nauplius, and in a more marked degree pos- 

 sesses some characters required in the theoretical crustacean 

 ancestor. 



