baymond: new and oll silurian trilobites. 39 



There is still a third possibility, namely, that Ceraurtis hydei is the 

 young of Cheirurus niagarensis, with which it occurs. On this third 

 point, Weller states that the fixed cheeks of C. hydei lack the pitted 

 surface characterizing C. niagarensis, and that C. hydei has a border 

 all around the cephalon, while C. niagarensis lacks it in front of the 

 glabella. These facts seem to be borne out by the type which is now 

 before me, and Professor Weller might have added that the glabella 

 expand more rapidly in the young of C. niagarensis than in C. hydei, 

 and has deeper glabellar furrows. The eyes too, of the young of C. 

 niagarensis are much further back than those of C. hydei. 



Against these differences we may, however, place the fact that the 

 thorax is alike in the two species, and more similar to the thorax of 

 Cheirurus insignis Bey rich than to any of the Ordovician species of 

 Ceraurus. In both Cheirurus niagarensis and Ceraurus hydei, the 

 part of each pleural lobe between the dorsal furrow and the fulcral 

 line is very much reduced, the diagonal furrow is very short, and the 

 two small nodes which it separates are narrow, and one directly in 

 front of the other, a point not brought out in Weller's somewhat 

 generalized figure. On the fulcral line there is a row of nodes, and 

 just inside this row is a longitudinal furrow parallel to the dorsal 

 furrows. Beyond the fulcral line, the pleura are free, not contiguous 

 as shown in Weller's figure. These same characteristics are shown in 

 two specimens of Cheirurus niagarensis from Wauwatosa, Wisconsin. 



On the whole, it does not seem very probable that C. hydei is the 

 young of Cheirurus niagarensis, especially as there is another species, 

 Ceraurus nuperu^ (Billings) which has a Cheirurus-like cephalon and 

 Ceraurus-like pygidium. The choice seems to lie between calling 

 it a Ceraurus or a Cheirurus. Theoretically, it would seem that the 

 Ceraurus pygidium was more specialized, and, therefore, less apt to 

 be dupHcated than the Cheirurus head. Most of the other Cheiruri- 

 dae, except Ceraurus, have all the spines of the pygidium approxi- 

 mately equal. 



In Ceraurus -pleurexantherrvm there is a tendency in some specimens 

 to have the basal lobes of the glabella triangular instead of square, 

 and in Ceraurus viisneri the glabella occupies a large part of the 

 cephalon, and the cephalon is long. Further, Ceraurus reaches the 

 climax of size and abundance in the Trenton, the late species being 

 smaller, and the specimens rarer. As to the thorax, I have shown that 

 this portion of the test changes in parallel directions in many lines of 

 the Asaphidae, and the same might well happen in the Cheirurudae. 

 On the other hand, one would not expect a decadent race to show new 

 characters similar to those of a race which is at its best. 



