534 bulletin: museum of comparative zoology. 



Ceraurus dentatus, sp. nov. 

 Plate 1, fig. 1; Plate 2, fig. 4, 5. 



Ceraurus pleurexanthemus Hall, (partim), Pal. N. Y., 1847, 1, pi. 

 65, figs. Id, lh, li, lm; pi. 66, figs, la-lg. Emmons, Geology 

 Second Dist. New York, 1842, p. 390, fig. 6; Amer. geol., 1855, 1, 

 pi. 15, figs, la, If, lh, li, Ik. Billings, Geol. Canada, 1863, p. 188, fig. 

 188. Cumings, 32d Ann. rept. Indiana state geol. surv., 1908, pi. 

 54, figs. 9-9b, (after Hall). 



The larger specimens figured by Hall differ from the typical form of 

 Ceraurus pleurexanthemus in having the eyes much further back and 

 without eye-lines, in the possession of much longer and less divergent 

 genal spines, and in having two pairs of well developed spines on the 

 pygidium, within the great spines. 



It might seem that these characters were merely the accompani- 

 ments of the greater size attained by the specimens, but that such is 

 not the case is shown by the fact that we have before us specimens of 

 C. dentatus from Canada which are smaller than the large specimens 

 from Trenton Falls. And, as has already been stated, series of speci- 

 mens from Trenton Falls show that there is no backward movement 

 of the eye or increase in the length of the genal spines with increase 

 of size. 



It is rather peculiar that the only good figure so far ascribed to 

 Ceraurus pleurexanthemus should prove to belong to this new species. 

 The figure referred to is the one given by Billings in the Geology of 

 Canada. The Museum of the Geological Survey of Canada does not 

 contain any specimen as perfect as is indicated by this figure, and it is 

 very probable that it is a composite illustration. The general form, 

 the cephalon, and pygidium would appear to have been drawn from 

 specimen No. 1,775, which is now selected as the holotype of Ceraurus 

 dentatus, while the thorax and ornamentation were apparently drawn 

 from specimen No. 1,769. The sharp pustules on the posterior border 

 of the cephalon especially are very like those on that specimen. The 

 figure is less than half the size of No. 1,775, but is almost exactly one 

 half the size of a third specimen, No. 1,769b. 



Another well known example of this species is the large specimen 

 figured by Hall on Plate 66 of the Paleontology of New York, 1. 

 This specimen was said by Hall to have come from " near Cincinnati, 

 Ohio," but it is entirely unlike any specimen which has been found in 

 that vicinity since, and very similar to the specimens found in New 

 York and Canada. A recent examination of this specimen in the 



