FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 73 



METHODS OF STUDY 



The identification of this large multituberculate collection and its 

 classification in genera and species have been peculiarly difficult, and 

 the work was accomplished, as far as it was possible at all, only after 

 prolonged and tedious analysis. Much of it led to no useful result and 

 so is wholly omitted here. An outline of the useful methods employed 

 ■will, however, be given, because they are similar to those used through- 

 out this v/ork and are in some parts unfamiliar to many paleontologists. 



In the first place, it was found that although at least three, and 

 probably four or five, genera are represented, it was not practical to 

 begin v^^ith a generic grouping of the specimens, as is the more usual 

 practice. The greater part of the collection consists of lower jaws. 

 With very few exceptions, which proved to be of little practical 

 assistance, the upper jaws all belong to a single species, and in only 

 one case are upper and lower jaws associated. In the family Ptilo- 

 dontidae the genera are usually readily distinguishable on the basis 

 of upper teeth, but in several cases, notably Ptilodus and Edypodus, 

 they are practically indistinguishable from lower teeth alone. The 

 only really clear-cut generic distinctions in the lower dentition so far 

 recognized depend on the nature of the incisor, whether compressed 

 and with limited enamel band or not, and in the presence or absence 

 of Ps.^^ In the present collection, only a few fragmentary specimens 

 (without cheek teeth) have an Eucosmodon-like incisor, and only one 

 P4 lacks the notch for P3. These characters are therefore not available 

 for the bulk of the collection, and it was necessary to treat the whole 

 collection as if it represented only one genus, to distinguish the species 

 present, and then to attempt to place them in genera. 



The following observations were made and recorded for each 

 specimen in the collection: 



IsTumerical: 



Dimensions: 



Length of P4. 



Length and width of Mi, M2, P', P^, P^, P^^M^ and M^. 

 Ratios: 



Length P4 : Length M,. 



Length Mi : Width Mi. 



Length Mi : Length M2. 



Width Ml : Width M2. 



Length Mj : Width M2. 

 Counts: 



Serrations of P4. 



Cusps of Ml, M2, P3, F\ Ml, and'M2. 

 Nonnumerical: 



Presence or absence of P3. 



Character of incisor. 



Shape of P4. 



>9 The peculiarly reduced P4 of Mkrocosmodon Jepsen is also a good generic distinction, but this strange 

 orm does not occur in the present collection. 



