82 THE CERATOPSIA 



at Yale was used for a basis of description, and comparison was made in so far as possible with the 

 type of Monoclonius crassus Cope, as described in considerable detail by Hatcher in the Ceratopsia 

 monograph. So far as the skeleton was concerned, no differences other than of specific rank were 

 discernible; without the skull, however, generic identity is difficult of establishment. The chief 

 distinctive characters of Monoclonius, as determined by Hatcher, 8 "are its small size, the diminutive 

 supraorbital horn cores pointing directly upward; the short, broad, and widely fenestrated parietals; 

 the short squamosals, as indicated by the squamosal border on the parietals in the type." 



The type of M. crassus does not include a nasal horn core, so that the character of this element 

 cannot be included in the generic description, as based upon that type. The horn of M. sphenocerus 

 is present, but is unassociated with a crest and therefore might as readily have pertained to a Styraco- 

 saurus, to which it has actually been referred (see beyond, p. 90). Hence, one cannot prove from 

 the original material whether this type of horn was characteristic of Monoclonius or not. 



The crest of M. crassus differs from that of Centrosaurus a-pertus in the presence of five prom- 

 inences, forming a median row as in Triceratops, and in the lack of any indication of the four posterior 

 horn-like processes which are highly diagnostic of every Belly River specimen pertaining to Lambe's 

 genus, except that one or the other of the forwardly-directed pair may be missing, as in Brown's 

 M. (C.) flexus type, but never both so far as I can ascertain. On the other hand, their presence 

 has never been demonstrated on any Judith River specimen. 



The long nasal horn and short brow horns; the short, deep muzzle; the saddle-shaped crest, 

 with its short squamosals and widely fenestrated parietals, the latter with a smooth median bar; and 

 the two pairs of processes arising from the rear, an anterior sulcated pair (one of which may be unde- 

 veloped) projecting forward to overhang the fenestrae, and the posterior curved, hook-like pair, 

 constitute the diagnostic characters of the sub-genus Centrosaurus. 



The nasal horn presents little difficulty, as it is not demonstrably absent from Cope's type, but 

 the processes of the crest are, to my mind, a serious obstacle to the acceptance of generic identity, as 

 they are demonstrably absent in the type of the genus. Geographically, the two genera come from 

 areas at least 250 miles apart; stratigraphically, the Belly River and Judith River formations are 

 stated to be equivalent (see chapter on distribution, p. 6). Be that as it may, the predentate 

 dinosaurs from the two areas present a very different aspect, and this refers to the unarmored 

 trachodons as well as to the ceratopsians; for the Belly River forms are often grotesque, such as the 

 helmeted dinosaurs culminating in the remarkable Parasaurolophus with the extreme backward exten- 

 sion of the skull, and Monoclonius {Centrosaurus) and Styracosaurus with equally remarkable 

 spinescence. That this condition may be due to racial abnormality having its seat in the pituitary 

 gland, which, as is well known, does react to certain peculiar conditions of the physical or chemical 

 environment, may have been true. Nevertheless, the specializations to which such conditions give 

 rise may be considered as having taxonomic value, whether generic or specific. It cannot be that, as 

 Nopsca and others have argued, these distinctions are sexual, for that would mean permanent segre- 

 gation of the sexes either geographically or in time, a manifest absurdity from the standpoint of 

 racial continuity. 



Monoclonius crassus Cope 9 



Holotype: No. 3998 A.M.N.H.; incomplete median element of crest. The sacrum and other skeletal elements, 

 described in detail in the Ceratopsia Monograph, bear the same catalogue number as the holotype (No. 3998), 

 although sometimes with a query. There are also a number of isolated vertebrae, with no record available 

 as to their associations although in size and general character they agree with the remainder of the skeleton. 

 No. 3997: prefrontal and postfrontal with brow horn, questionably referred to M. crassus. 



Horizon : Judith River formation. 



Locality: Nearly opposite the mouth of Dog Creek on the north side of the Missouri River, Montana, in the same 

 locality as M . recurv'icornis. 



Collector: E. D. Cope, 1876. 



8 Hatcher, Marsh, Lull, 1907, p. 70. 



9 Cope, E. D., 1876, pp. 255-256. 



