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136 ^ The Ottawa Naturalist. [Feb. 



has two long neck spines, and would be a Ceratocephala, while 

 the third has a single long neck spine, and ^vould have to be 

 called Acidaspis. Except for these spines, the species show no 

 important differences, and it is evident that in any natural 

 classification they would be congeneric. Compared with the 

 type-species of Ceratocephala and Acidaspis, Acidaspis dujrenoyi 

 and .4. hoernesi show marked differences in all parts except in 

 the spines on the occipital ring. 



Ce)-aiocephaIa, Warder, Am. Jour. Sci. 34, 1838', p. 377. Type, 

 C. goniata, ibidem, p. 378, fig. The typical species was badly 

 described and figured by Warder, but all parts are now known. 

 Among the striking features of this trilobite one may note the 

 coalescence of the free and fixed cheeks, accompanied by the 

 obliteration of the facial stiture, the almost complete oblitera- 

 tion of the dorsal furrows on the cephalon, and the position of 

 the eyes, far from the glabella, and half way to the front of the 

 cephalon. On the thorax the horizontal furrow on the pleural 

 lobe of each segment is weak, and the two low ridges separated 

 by this furrow are equal. The pygidium has long subequal 

 barbed spines. 



Acidaspis, Murchison. Silurian System, 1839, p. 658. 

 Type, .4. hrighii Murchison, ibidem, pi. 14, fig. 15. The glabella 

 of the typical species is roughly triangular in outline, tapering 

 rapidly forward. The eyes are situated far back and close to 

 the glabella, and the whole neck ring is prolonged backward 

 into a long heavy spine. No more than the cephalon of the 

 typical species'is definitely known. In the American .4. anchor- 

 alis and ,4. onealli, which have the same sort of a cephalon, the 

 thoracic segments are narrow, and the linear horizontal furrow 

 separates a high narrow posterior ridge from a low narrow 

 anterior one on the pleural portion of each segment. In these 

 same species, the pygidium has two long lateral spines, between 

 which are short spines, and outside of which are small spines. 

 A similar pygidium has been referred to .4 . brighii. 



Odontopleura, Emmrich. De Trilobitis, 1839, p. 53. Type, 

 0. ovata Emmrich, ibidem, pi. fig. 3. The type, an entire speci- 

 men, is characterized by its broad form, an oval glabella which 

 does not taper much toward the front, and the central position 

 of the elevated ridge on the pleural lobe of each thoracic seg- 

 ment. The pygidium is not unlike that ascribed to Acidaspis, 

 except that the spines are more nearly equal in size. 



As one looks over the various Odontopleuridae* which have 

 been described, it is seen that there are a few which agree with 

 the type of Ceratocephala in having the fixed and free cheeks 

 in symphysis, eyes well forward, and pleura of thoracic seg- 



