104 



BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



significantly more than E. a. 'primus. The minimum diameter is about 

 that of E. teilhardi, but the maximum diameter and the compression 

 are significantly less. E. americanus and E. molestus have the incisors 

 significantly larger in both diameters and the compression less. Exact 

 comparison with E. gratus Jepsen is not possible, but the incisor 

 alveolus of that species measures 2.7 by 1.1 mm, ratio 2,45. The 

 alveolus, and of course therefore the incisor as well, are much smaller 

 than the incisor of E. sparsus. No known species aside from the 

 several now mentioned could well be so closely related to E. sparsus 

 as to demand differentiation. 



Table 21. — Comparison of data on lower incisors of Eucosmodon 



Species and specimen 



Diameters 



Maximum Minimum 



Ratio 



E. american us, type 



E. a. primus, paratype 



E. molestus, neotype 



E. teilhardi, paratype 



{Means 

 10113 

 9705 

 9S6lII-l--III--I" 

 {E. gratus, type, from alveolus) 



Mm 



6.0 



4. 2 



5. 8 

 5. 2 

 4. 3 

 4. 

 4. 1 

 4.0 



(2.7) 



Mm 



2. 7 



1. 9 



2. 6 

 1. 6 

 1. 5 

 1. 4 

 1. 6 

 1. 5 



(1. n 



2. 22 



2. 21 



2. 23 



3. 25 

 2. 70 



2. S6 

 2. 56 



2. 67 



(2. 45) 



The original of Douglass' plate 1, figures 18 and 20, probably belongs 

 to this genus. If the scale of the figure is accurate (which is not 

 invariably true in cases where I have studied Douglass' originals), the 

 maximum diameter is about 3.7 mm, smaller than other specimens 

 referred to this species, to which the specimen may nevertheless 

 belong. 



Order INSECTIVORA Gray, 1827 



Forms that are at least nominally referable to the Insectivora con- 

 stitute an important element in Paleocene and Eocene faunas, and 

 this is particularly true in famias, like that here described, in which 

 the microfauna is well or disproportionately represented. They are 

 abundant in the present collection, including at least 10 species, all 

 of which occur together in the Gidley Quarry. 



Despite diverse specializations, such as in the incisors of the Sori- 

 coidea or the limbs of the Talpoidea, the recent insectivores are on 

 the whole the most primitive of living placental mammals. The 

 characters common to all of them, excluding the speciahzations of 

 various particular phyla or larger groups, are in general those unques- 

 tionably primitive for all placental mammals. The conception of 

 the order and its definition thus do not so much depend on special 



