198 Workman — Studies of Eocene Mammalia in the 



least two generic types living in this country during the depo- 

 sition of the Bridger sediments, the sum total of whose char- 

 acters, as far as they can be at present ascertained, come nearer 

 to the living Yiverridae than to any other known group of the 

 Carnivora. These characters, moreover, not only do not 

 interpose any difficulties in the way of deriving the one group 

 from the other by direct descent, but they furnish just such a 

 combination of resemblances and differences as we should 

 be reasonably led to look for on a priori grounds, in the 

 Eocene ancestors of the existing Yiverridse. The living repre- 

 sentatives of this family include three and perhaps four widely 

 divergent types, so different from each other that at various 

 times they have been considered by some very good authorities 

 to represent distinct family modifications. One of these, the 

 Yiverringe, for which there are excellent reasons to believe 

 that they come nearest to the original stem form, has fossil rep- 

 resentatives in the Upper Eocene of Europe, whose remains are 

 indistinguishable generically from those living to-day. This 

 fact indicates a most remarkable persistence or stability of 

 structure which we are not at liberty to suppose began 

 abruptly in the Upper Eocene, but must have, in some degree 

 at least, belonged to its earlier Eocene progenitors. JSTow it is 

 either a striking coincidence or a highly significant fact that this 

 very character is one of the preeminently distinguishing features 

 of Viverravus. We have already seen that it passes with but 

 very little change from the Torrejon to the Bridger inclusive, 

 or, in other words, it did not change more than specifically 

 during the deposition of between six and seven thousand feet 

 of sediment. There is another fact which must not be lost 

 sight of just here, and that is, that in the Eastern Hemisphere 

 these forms appear abruptly in the upper stages of the Eocene, 

 while in the Western Hemisphere they disappear quite as sud- 

 denly in the age preceding. So much, then, for the general con- 

 siderations touching the possible relationship of the two groups. 

 Let us next inquire into the special or particular resem- 

 blances and differences which they present. In the Viverravus- 

 Viverra series we note, (1) that the dental formula is 

 identically the same in the two groups ; (2) that the structure 

 of the various teeth, including all the details of arrangement 

 with respect to the component cusps, ridges, etc., is surpris- 

 ingly similar ; (3) that the skull of Viverravus exhibits the 

 same more or less compressed, post-orbitally elongated form as 

 that of the Yiverringe ; (4) that the atlas has the same arrange- 

 ment of the perforations for the passage of the vertebral 

 artery — a highly characteristic viverrine feature ; (5) that 

 the succeeding vertebrae are very similar in the two groups, 

 including the long and powerful tail, which is still retained ; 



