1910.] Origin of the Insectivora. 287 



(9) Angle of lower jaw inturned: moderate in Iciops, sliglit in most 



species of Erinaceus, pronounced in E. deserti, E. algirus (Dobson, 

 1882-83, p. 37). Never approaching the characteristic Marsupial 

 shape. 



(10) Posterior mental foramen beneath m^: ZaUimbdodonts, Pantoleates, 

 M yog ale. 



(11) Nineteen dorso lumbar vertebra' : Tupaia varies however from 18 to 

 20; Macroscelides, 19-20; Rhynchocyon, 21. 



Some of the foregoing characters may well have been acquired inde- 

 pendently in the different phyla and all of them indeed never occur in the 

 same form, but as a whole thesy are reinforced by the remarkably detailed 

 "Marsupial" features in the brain and Jacobson's organ of Macroscelides. 

 The conclusion is: that in addition to the large number of primitive Marsu- 

 pio-Placental characters which Insectivores share with other lowly Placentals 

 there is also a considerable number of special "INIarsupial" resemblances, 

 some of which may also be of primary and not secondary derivation, and 

 which, taken as a whole, indicate that the order Insectivora is structurally 

 nearer to the Marsupio-Placental stem than is any other existing Placental 

 order. 



Speculations on the Origin of the Order Insectivora. 



So little is known about early Tertiary Insectivores (except the Leptic- 

 tid« and certain other groups) that speculations on the origin and history 

 of the group must be based largely upon existing structures and conditions, 

 and consequently the difficulty of distinguishing paheotelic from ctenotelic 

 or adaptive conditions confronts us at every turn. But after considering 

 numerous contrary hypotheses the writer tentatively adopts the following 

 as being on the whole the most likely phylogenetic interpretation of the facts 

 and analyses which have been assembled in the preceding pages. 



According to this hypothesis the characters of the family of mammals 

 which gave rise to the modern order Insectivora may be conceived as follows : 



(1) Tim,e of appearance pre- Tertiary. The occurrence of well differ- 

 entiated Zalambdodonts and Erinaceoids in the Lower Oligocene, and of 

 Leptictids, Soricoids, ]\Iixodectids, Pantolestids and possibly Tupaiids 

 (p. 285) in the Eocene, joined to the frequency of "Marsupial" characters 

 in modern Insectivores, the wide interval between the Lipotyphla and Meno- 

 typhla, and the generalized Placental characters of the order as a whole, 

 all indicate a relatively great antiquity for the stem form, as compared with 

 other Placental orders, some of which probably ran l)ack into the Upper 

 Cretaceous. 



