MORRISON MAMMALS: MULTITUBERCULATA 25 



Skull 



The root of the zygoma is stout and arises opposite P'*. The arch was widely 

 expanded and the anterior part was formed by the maxilla. The infraorbital foramen 

 is small and is situated low down just in front of, or indeed on, the zygomatic root. 

 Consonant with the arrangement of the molars, the posterior part of the alveolar proc- 

 ess of the maxilla curves inward abruptly back of M\ Much of the palate may be repre- 

 sented by flakes of bone, but its characters are destroyed. 



Psalodon Simpson 1 926 

 1926. Psalodon, Simpson, Atner. Jour. Set. (5) XL 239. 



Definition.— P' with three cusps, but one much the largest and one vestigial. 

 P*"» more highly developed for shearing, more compressed laterally, with outer cusps 

 reduced in number and vestigial in character. Lower jaws, doubtfully referred, close 

 to those of Cteuacodon but considerably larger than any known species of the latter. 



7ype. — Psalodon {Ctenacodon) fotens (Marsh). 



Distribution. — Morrison, Wyoming. 



The circumstances which led Marsh to refer this type of upper jaw to Cteuacodon 

 have been given in dealing with Ct. laticep. The upper jaws here referred to Psalodon 

 do differ from those referred to Ctenacodon sufficiently to follow Marsh in considering 

 them as representing different genera. 



If the lower jaws here referred to as "> Psalodon marshi do belong to Psalodon, 

 they illustrate a multituberculate peculiarity which has been noticed also in dealing 

 with later forms; that is, that the upper teeth may show very marked qualitative differ- 

 ences although the lower jaws at any given time may be, as it were, stereotyped, differ- 

 ing only in size and proportions. This has been seen in several Paleocene forms and is 

 apparently equally true of the Cretaceous multituberculates. Ultimately the chief 

 taxonomic characters will be found in the upper jaws, although at present associated 

 material is too scanty to follow this method consistently. 



To Psalodon are now referred the upper jaws on which Marsh based his Ctenaco- 

 don fotens, the lower jaws referred to this species by Marsh, and the premaxilla called 

 Allodon fortis by him. Because of the doubtful association it seems best to retain both 

 trivial names, fotens and fortis, and to refer the lower jaws to a third species, Ps. 

 marshi with the understanding that these names are adopted provisionally for con- 

 venience of reference and of cataloguing and that future discoveries may prove them to 

 be synonymous. 



