PORT UXIOX OF CRAZY MOUNTAIN FIELD, MONT. 97 



An isolated P4 from Loc. 50 (American Museum collection) measiires 

 6.1 mm in length, has 15 serrations, and closely resembles the type of 

 this species in form. There can be little question that it belongs here, 

 as it is so close to the known mean for this species and far outside the 

 range of any other species recognized in this field. 



7PTILODUS SINCLAIRI Simpson 



Figure 9, c 



tPlilodus sinclairi Simpson, 1935d, p. 225. 



Type.— U.S. ^.M. no. 9770, left lower jaw with P4-M2. Collected 

 by A. C. Silberhng. 



Horizon and locality. — Gidley Quarry (referred specimens from 

 Silberling Quarry), Fort Union, Middle Paleocene horizon, Crazy 

 Mountain Field, Mont. 



Diagnosis. — Length P4, mean 3.1 ±0.07, standard deviation 

 0.29 ±0.05. Length Mi, mean 1.9 ±0.03 standard deviation 0.08. 

 Ratio length P4: length Mi, mean 1.6 ±0.08 standard deviation 0.22 ± 

 0.06. Ratio length Ml : wddth M], mean 2.2 ±0.06, standard devia- 

 tion 0.17±0.04. Serrations P4 10-13, m.ode 12. Cusps Mi 6-7:4, 

 mode 7: 4. 



Remarks. — Some of the peculiarities of this species, second only to 

 P. montanus in abundance, have already been discussed above. Its 

 very small size at once distinguishes it from any species of related 

 genera except lEdypodus silberlingi (diagnosed below), and Parecty- 

 podus tardus, with which it cannot be congeneric, as it has P3. 



The two specimens (6089 and 6090, and also a third, 6149, so labeled 

 but not published by number) that Gidley at first (1909, p. 623) re- 

 ferred to "Ptilodus formosus'^" (Marsh) {=Halodon jormosus Marsh) 

 belong to IPtilodus sinclairi.*^ Adequate comparison with the frag- 

 mentary Cretaceous types of Marsh is impossible, but in view of the 

 very different age and of the fact that when close comparison is pos- 

 sible not only the species but also the genera are very distinct, it may, 

 I think, be assumed that the Fort Union forms do not belong to 

 Cretaceous species. 



As noted above, it is improbable that this species belongs to Ptilodus, 

 but it cannot at present be clearly distinguished from that genus. 



«5 Gidley did not change their labels, but it is practically certain that he recognized their pertinence to a 

 distinctive species. He recognized tPlilndun sinclai-i (under a different, unpublished name), and so labeled 

 about half the specimens that I place here, covering almost the same ranse of variation, so that in this ease 

 his specific criteria and mine lead to nearly the same result. Ee also included, however, one or two speci- 

 mens that I place in other species. 



