OLENELLUS AND OTHER GENERA OF MESONACID.E 



249 



DEVELOPMENT OF MESONACID/E 



The development of the JMesonacidse from some anneUdian-Hke 

 ancestor by the gradual combination of segments to form the cepha- 

 lon and pygidium is indicated by the examples cited of Nevadia, 

 ElliptocepJiala, Holmia, and Pccdeiimias. The cephalon, as we know 

 it, was developed in pre-Cambrian time, also the pleural lobes of the 

 thorax. The compact, strong pygidium, made up of manv seg- 

 ments, does not occur in the Mesonacidse, and is unknown in any 

 trilobite from the beds of the Lower Cambrian in which the simplest 

 form of the Lower Cambrian trilobites {Nevadia zveeksi) occurs. 



With my present information I am inclined to think that Paradox- 

 ides descended through the CaUavia-Wanneria line, rather than the 

 Mesonacis-OleneUus line. The latter line expended its force and 

 became extinct in Lower Cambrian time, leaving no descendant to 

 pass into the Middle Cambrian. 



Diagrammatically represented my present conclusion as to the 

 development of the known genera of the Mesonacidae is as follows, 

 beginning with Ncz'adia at the base. 



The presence of a //o/wi/a-like species {H. rowei) with Nevadia in 

 the oldest known horizon of the Mesonacida^ indicates that more 

 primitive forms of the Nevadia type existed at an earlier epoch 

 before Holmia was developed by the absorption of the simple pos- 

 terior segments of its Nevadian progenitor. 



