MORRISON MAMMALS: MULTITUBERCULATA 



17 



Type of Ct. nanus.— Cat. No. 1 1832, Y.P.M. Left lower jaw with P1.4 and M,-2. 



Princiiwl Referred Specimens. — Cat. No. 13668, Y.P.M. Left lower jaw in 

 matri.x, inner aspect, complete save for tip of coronoid, incisor, and molars. Fig'd, 

 Marsh 1887. PI. VII, fig. 4. 



Cat. No. 2688, U.S.N.M. Lower jaw with P1-4 well preserved and little worn. 



Horizon and Locality. — Morrison formation, Quarry 9, Como Bluff, Wyo- 



ming. 



Diagnosis. — Length Pi-4, about 4.2 mm. Length ratio P4: P3, 1.54. Three serra- 

 tions on edge of P3 and six on P4. P4 lower relative to its length than in C. scindens. 

 Anterointernal cusp of Mi small and not very distinct. 



The following characters were supposed by Marsh to be distinctive of his species 

 C. nanus: 



1. "The last premolar only has its summit marked by faint notches." 



2. "The ridge starting at the base of the coronoid and extending forward on the 

 outer side is much sharper than in the larger species." 



3. The following comparable measurements are given: 



Space occupied by P1-4 

 Height of P4 



The first character was not given diagnos- 

 tic value, and reexamination shows that it is 

 accidental, the summit of P3 being broken in the 

 type of C. nanus. The second character is not 

 confirmed by simultaneous study of the two spe- 

 cimens under the binocular microscope. New 

 measurements (see below) show the difference 

 in size of teeth to be so slight that it certainly is 

 not of specific value in itself. The jaw of C. 

 n^tmts is about 13 per cent smaller, but this is 

 due in large part to its being a younger individ- 

 ual, possibly also to sexual difference. The gen- 

 eral agreement is so close that both species cannot be maintained. 



Dentition 



The dental formula is clearly Ii Co P4 Mo. The incisor is not known in associa- 

 tion, but, from its alveolus, it was a simple, large, curved tooth of oval section. It is 

 followed by a short diastema back of which are the four premolars, forming together 

 a compressed wall. The first premolar hardly joins in the shearing edge. It is small 

 and low and rises to a single point which nearly touches the somewhat overhanging 

 anterior portion of the succeeding tooth. It is implanted by two subequal roots and has 

 a swollen base, almost circular in horizontal section. 



The second and third lower premolars are much alike, save that the third is 



CTENACODnN 



Fig. 5. Ctetmcodon. Diagrammatic inter- 

 nal view of right lower jaw. Three times 

 natural size. 



