22 AMERICAN MESOZOIC MAMMALIA 



the premaxilla did belong to Allodon, and in the second place, it has since been found 

 that the type of P in this jaw does not require a rodent-like lower incisor and that the 

 referred lower incisor is simply a broken tooth of the same type as that found in Ctena- 

 codon, Plagiaulax, and probably all plagiaulacids. Marsh further found support for 

 his position at that time in the existence of large lower jaws with normal incisors. He 

 naturally referred these to Ctenacodon {Ct. fotens), being of the opinion that they 

 could not belong with the premaxilla referred to Allodon. He also found large upper 

 jaws, suited in size to belong either with "Allodon" fortis or with the large lower jaws 

 referred to Ctenacodon. Since they differed from "Allodon" laticefs in generic charac- 

 ters, he naturally referred them to Ctenacodon, basing the species Ct. fotens upon 

 them. This conclusion, apparently logical at the time, also had a double weakness, 

 however, for it was not true that the large lower jaws could not go with the premaxilla, 

 and it was not true that the premaxilla necessarily belonged in the same genus as 

 "Allodon" laticefs. Hence there was, actually, no reason why these three parts of large 

 dentitions, of a size suitable to represent a single species, should not have been placed 

 together. In fact all later work seems to indicate that they did belong together. 



Marsh himself had found that the original generic character supposed to separate 

 Allodon and Ctenacodon was incorrect, and it has just been shown that the additional 

 evidence submitted in 1887 also failed to indicate that Allodon and Ctenacodon were 

 distinct. "Allodon" laticefs is suitable in every way to be the upper dentition of Ctena- 

 codon serrattis. The teeth occlude perfectly, and there is now no evidence against this 

 view. Until upper and lower teeth are actually found in association, it is convenient 

 and justifiable to retain Marsh's trivial name laticefs, but there can be little doubt that 

 it should be referred to Ctenacodon. 



Turning from these relatively unimportant matters of nomenclature, it will be 

 seen that Marsh was in 1887 many years ahead of contemporary thought in his con- 

 ception of the true characters of the plagiaulacids, for he insisted that the upper jaws of 

 Allodmi type were closely related to the lower jaws of Ctenacodon type. Cope and later 

 Osborn, on the other hand, placed these upper and lower jaws in different families, 

 Plagiaulacidae for the lower and Bolodontidae or Chirogidae for the upper teeth. So 

 cogent were the arguments which they advanced that Marsh himself finally abandoned 

 his correct view and proposed a family Allodontidae, as already mentioned. It was 

 not until Gidley's description of the full dentition in Ptilodus (1909) that Marsh's 

 first opinions were fully vindicated. 



Dentition 



The dental formula is V^ C" P^ M^. This formula, although often accepted, has 

 been subjected to much criticism and requires defense. The incisors are not known, but 

 they must have been much as in Bolodoti or Psalodon, both of which have three pairs. 

 Marsh stated that there was in the type (Y.P.M. No. 11761) an alveolus anterior to 

 the preserved premolars in the maxilla and lodging originally either another premolar 

 (making six) or a canine. There is such an alveolus, but it is strongly inclined for- 

 ward and the alveolar border is here broken away. I f the alveolar border were restored 



