FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 87 



I^ is apparently present in only one specimen, the most complete 

 one, no. 6076, and here I suspect that it is incorrectly shown. The 

 tooth inserted in this position is an incomplete crown, with no root, 

 bedded into the broken I^ alveolus in plaster. In size and structure 

 it is exactly a mirror image of the same part of the I^ of the same side, 

 right, which seems almost conclusive evidence that it is, in fact, the 

 left I^ that was loose in the matrix and was erroneously inserted in 

 this position. If this is true, I^ is still unknown in this species.^^ 



I^ and I- were well spaced, and another diastema of about equal 

 length lies between I' and P^ The latter tooth is tricuspid in all 

 specimens, with tliree equal cusps, one anterior and a transverse pair 

 posterior. P^ is also constant in cusp structure and resembles P^ but 

 is wider and quadrate, with four cusps, two transverse pairs. P^ is 

 narrower than the adjacent teeth and as shown elsewhere is extremely 

 variable in size and in construction. There is little doubt that it is 

 in process of reduction and that its great variability is a feature of 

 degeneration.^^ The cusps are similar to those of P^~^ but smaller and 

 more variable. There are always two transverse pairs, and on the 

 bulging anterior and posterior basal parts others may be developed. 

 When present, these are usually anterior, one or a pair, but in two 

 specimens there is also a cusp posterior to the constant four. 



P^ the upper shearing tooth, is much enlarged and has a plane, 

 slightly inclined inner face. The internal cusp row consists of numer- 

 ous small cusps (full data on cusp number are given elsewhere), united 

 nearly to their apices and arranged in a straight anteroposterior hne. 

 External to this is another row, slightly shorter posteriorly, somewhat 

 curved (convex externally), with fewer, larger, and more separated 

 cusps. On the anterior part of the external face of the tooth the base 

 bulges outward, forming a shelf which is usually vaguely papillate, 

 without distinct cusps, but may have one or more cusps. 



The premolar cusps are all similar, nearly conical, sharply pointed, 

 the enamel furrowed and ridged radially from the point, one to three 

 of these ridges developed into more prominent, sharp crests. 



M^ has three cusp rows, and these are of nearly equal length when 

 unworn. The internal row, however, narrows anteriorly. All its 

 cusps are relatively smaller, and anteriorly they become numerous and 

 minute. This anterior part may be shorter than the other rows, but 

 it is invariably sheared off very early in life by backward movements of 

 P4. The middle and outer cusp rows are of equal width and cusp size 

 throughout, and the two are of about equal length. The cusp form is 



38 The error, if such it be, is a very natural one, especially as the tooth may have lain near the alveolus 

 since the fragments of this specimen are very much disturbed in the matrix. Gidley notes the resemblance 

 of the tooth to I' except for the apparent reversal of inner and outer sides. His statement that it is smaller 

 is true only of the apparent height, and the tooth is brolcen and the true height not shown. 



3' Reduction in the midst of the premolar series rather than at its ends may be characteristic of m.ulti- 

 tuberculates. In the Plagiaulacidae of the Jurassic, it seems to be the third of the five premolars that is 

 being reduced, and hence the ptilodontid premolars may be P'"' and P*"' of the ancestral series. 



119212—37 7 



