106 BULLETIN 169, UNITED STATES NATIONAL MUSEUM 



The concept Insectivora as it is applied to the Paleocene (and 

 Eocene) faunas may thus inchide four different sorts of lesser groups: 



1. Very primitive placentals whose ancestral relationship to other 

 later groups is not now recognizable. Naturally we do not know 

 what groups may be included here, for the only way in which this 

 situation could be shown to exist would be by recognition of the rela- 

 tionships, but probably some so-called insectivores are of this char- 

 acter. 



2. Animals that are in fact in or near the ancestry of later more 

 specialized insectivores and that are therefore Insectivora sensu 

 stricto. The Nyctitheriidae probably belong in this category, al- 

 though the relationship is not definitely established. The other 

 famiUes in the present fauna almost surely are not Insectivora in 

 this special and most limited sense. 



3. Animals not structurally representative of the ancestry of later 

 insectivores but sharing certain specializations with them that seem 

 to indicate that they arose from a common stock with the later groups 

 after the definite differentiation of that stock. These are also to be 

 considered as Insectivora in a strict sense, even though the usage is 

 broader than it is as apphed to the second category. The degrees of 

 such collateral relationship vary greatly. Thus the Leptictidae fall 

 definitely in this category, as they share many and apparently sig- 

 nificant special characters with the Erinaceidae, although clearly not 

 ancestral to that family. The Pantolestidae likewise show evidence 

 of a special collateral relationship to the later insectivores, but the 

 resemblance is less particular and the relationship evidently more 

 remote. 



4. Groups that were derived from the nominally insectivore pro- 

 toplacental stock but that had begun to diverge markedly from any 

 other groups, without, however, having a sufficiently long history, 

 being sufficiently important faunal elements, or acquiring sufficiently 

 striking special characters to warrant the erection for them of a 

 special order. Such groups are clearly Insectivora only in a very 

 broad sense, yet their exclusion from the order would be a needless 

 complication of taxonomy. The Mixodectidae appear to belong to 

 this category. 



In this fauna there are 10 genera that are referable to the Insectivora 

 in the general sense here accepted. Gelastops is evidently related to 

 Didelphodus of the Lower Eocene and Acmeodon of the Middle 

 Paleocene. It is perhaps a modified survivor of the protoplacental 

 stock. Prodiacodon and Leptacodon are tj'^pical leptictids, Myrmeco- 

 boides is an aberrant member of that group, and Bessoecetor is a primi- 

 tive but typical pantolestid, while Aphronorus constitutes with the 

 Torrejon Pentacodon a more aberrant group probably of pantolestid 

 origin. Eudaemonema seems surely to be a mixodectid, although 



