FORT U:MIO]Sr of crazy mountain field, MONT. 



67 



Ptilodus or to Ectypodus are really congeneric in each case. There 

 may well be one or more other genera represented here, although it 

 does seem unlikely that each species could belong to a different genus. 

 Aside from this possibility, the species referred to one genus are in 

 every case sharply distinct from each other, with no intergradation 

 shown. This is true of many different characters, and is well shown 

 by table 5 of observed ranges of a few numerical characters: 



Table 5. — Ranges of numerical characters in multituberculates, Gidley Quarry, 

 Crazy Mountain Field, Mont. 



Genus and species 



Ptilodus: 



montanus 



douglassi. 



gidkyi 



sinctairi.. 

 Ectypodus: 



grangeri.. 



russelli... 



silbcrlingi 



LP4 



Mm 

 7. 1-9. 1 

 6. 5-6. 8 

 6. 9-6. 1 

 2. 5-3. 7 



5. 2-5. 4 



4. 9-5. 1 



3.3 



LM, 



Mm 



3. 2-3. 7 

 3.7 

 2.5 



1. 7-2. 



3. 3-3. 4 

 2.9 

 2.3 



LP4:LM, 



2. 3-2. 4 

 1.8 

 2.4 



1. 3-2. 



1. 5-1. 6 

 1.7 

 1.4 



Serrations 



P4 



13-15 



13 



14-15 



10-13 



13-14 



13-15 



12 



Cusps Ml 



5-6:4-5 

 6:4 



6-7:4 



10-11 

 9-10 



6-7 



6 



5-6 



Thus these species probably represent more than two genera and in 

 any case are so sharply distinct that each must have had its own 

 ecologic niche. They do not intergrade, but in some cases, notably 

 P. montanus, there are known species, in this case P. mediaevus, 

 with which they do nearly or quite intergrade but which did not live 

 together with them. In short, these are not exceptions to but are 

 strildng exemplifications of Cabrera's law. 



Leptacodon is here credited with two species. Their tooth dimen- 

 sions do not intergrade, although the degree of variation is well 

 established for one of them, and there are structural differences such 

 that they may well prove to be distinct subgenerically, or even 

 generically, when both are more completely known. This is even 

 more strikingly true of the two species referred to Paromomys, which 

 are so different that I was for a time inclined to separate them 

 generically (see fig. 3, b). Didymidis also is represented by two 

 species perhaps not really congeneric, not intergrading at all and one 

 reaching a size nearly twice that of the other. 



In the case of Claenodon, Gidley has been followed in listing three 

 species from the Gidley Quarry, all of about the same size and general 

 character. Differences between them do exist, but the material is 

 inadequate to establish the extent of variation, and when this is 

 established I suspect that the supposed three species will prove to be 

 variants of a single species. If not, this will be the unique example 

 of the occurrence of two or more closely related and apparently 

 ecologically similar species in the quarry. 



