PALEONTOLOGIC CONTRIBUTIONS 125 



might ultimately develop a more instead of a less spinose pygidium. 

 Ceraurus pleurexanthemus, as it occurs in the Tren- 

 ton, holds an intermediate place in the time occupied by the history 

 of the Cheiruridae. Older and more primitive genera like Pliomera 

 and some species of Cyrtometopus show a pygidium with several 

 pairs of subequal' spines, and later species, like some of the Silurian 

 species of Cheirurus, also show pygidia with a series of approxi- 

 mately equal spines. 



Does the pygidium of Ceraurus pleurexanthemus 

 then represent the end of a line in which there is a reduction from 

 a primitive species in which the pygidium has equal spines at the 

 extremities of the pleura, of all four segments, or is it in itself a. 

 primitive form, from which a many-spined pygidium might be 

 developed ? 



From the ontogeny of C. pleurexanthemus itself we 

 learn nothing. Although specimens only 3 mm long have been 

 studied, they do not differ from the adult. 



The pygidium of only one Ceraurus from a younger formation 

 than the Trentbn is known. That is the Ceraurus miller- 

 anus Miller and Gurley, of the Cincinnatian. In that species the 

 second pair of segments in the pygidium give off blunt spines, so 

 that the posterior margin is undulatory, and thus what slight varia- 

 tion exists is in the direction of the development of spines. The 

 only other indication in this direction is seen in Cheirurus 

 hydei Weller (Chicago Acad, of Sci. Bui., 4, p. 2, 1907, page 

 264), a form which might be considered as a sort of a bridge from 

 Ceraurus to Cheirurus, as it has the cephalon and thorax of a 

 Cheirurus and a Ceraurus-like pygidium. From other lines of 

 evidence however, it does not seem probable that this is a primitive 

 but rather a specialized Cheirurus. 



Turning back to the other hypothetical line connecting C . 

 pleurexanthemus with the supposed ancestor, there seems 

 to be somewhat better evidence, though the line is still very 

 incomplete. 



As has already been stated, the pygidium of C. dentatusis 

 less specialized than that of C. pleurexanthemus, as the 

 posterior margin between the. great spines has two pairs of short 

 spines and a trace of a median spine. C. dentatus is not, 

 how'ever, found in older beds than C. pleurexanthemus, 

 but its history is practically contemporaneous with it. Ceraurus 

 ruedemanni shows a very decided step back toward the many- 

 spined pygidium, the second pair of spines being strongly developed, 



