1 !)!().] (Jhiiractcr.s of the Stem hmctivores. 289 



part of tlie digestive tract from becoming overspecialized/ Nor does the 

 mixed fare require very i)()\verful masticatory muscles, the development of 

 which frequently occasions profound changes in the skull and dentition of 

 carnivorous and herbivorous mammals. 



(3) Size small, perha])S no bigger than that of HyJomijs or Microgalc. 

 The larger Centetidie (p. 236) are probably less primitive than the minute 

 Microgalc, while one of the larger of the living Insectivores Potamogalc is 

 also highly specialized (p. 240). The larger P^rinaceinae may also lead back 

 through the small Gymnura and Hyloniys to a very small Leptictid such as 

 Ictops thomsoni. 



There is also considerable indirect evidence that very small size is an 

 ancestral character in many groups of mammals: c. g., (a) The most 

 ancient forms reputed to be mammals (Dromatheriiim, etc.) are of extremely 

 small size; (5) The Jurassic Trituberculates (p. 177) are all of very small 

 size and certainly primitive in other respects also, (e) iVmong Marsupials 

 the smallest forms, Prnfcodidelplujs (p. 206), Pcrathcrium (p. 205), Marmosa, 

 Phascologale (p. 203), etc. are also believed to be the most primitive in other 

 respects, (d) Among Tertiary Ungulates and Carnivores the various 

 lines have been demonstrated, in every case known, to lead back to relatively 

 small forms. 



(4) Snout tubular more or less elongate, probably with a stout cartila- 

 ginous skeleton. This character is shown in nearly all existing Insectivores 

 in various degrees of development. It seems to be correlated with the 

 insectivorous diet and to be a development of the less emphasized projecting 

 type of muzzle which is preserved in Didelphis. In the Insectivores it is 

 also accompanied by the following character. 



(5) Premaxillaries more or less elongate, upper incisors arranged in an 

 anteroposterior series. This very early became a distinctive character of 

 the Insectivores and in its fully developed stage serves to distinguish them 

 from the ancestral Primates and still more from the ancestral Creodonts. 

 This condition was very likely derived from the intermediate condition 

 preserved in Didelphis, Eocene I^emuroids and Pantolestids, in which the 

 opposite incisor series are neither transverse (as in Carnivores) nor antero- 

 posterior in position, but converge more or less obliquely toward the middle 

 line. I^ may have been slightly enlarged and ij procumbent. 



(6) Canines small, single rooted. It might at first be thought that the 

 more or less double rooted, premolariform canine which is realized in 

 Microgale and which recalls the doul)le rooted canine of certain Jurassic 



1 Chalmers Mitchell (1905) regards the extreme simpUcity of the digestive tract in the 

 exclusively insect and worm eating forms as a secondary specialization. Only the Menotyphla 

 retain a more normal mammalian development of the different parts, including the cajcum. 



