426 Bulletin American Museum of Natural History, [^'ol. XXVII, 



and in Peltephilus among Edentates extending below the level of the cheek 

 teeth; in Marsupials and most Placentals above that level. In Insectivora 

 the postglenoid process appears to have been originally of small size, and 

 continuous with the "tympanic" ridge of the alisphenoid which runs in- 

 ward and joins the pterygoid ridge of the alisphenoid (p. 244); by the reduc- 

 tion of this ridge and of the process itself (in Macroscelides and Ptilocercus) 

 the glenoid fossa becomes more or less elongate and makes some approach 

 toward the conditions seen in Rodents (p. 331). In certain Rodents {e. g., 

 Lepus) the glenoid region shifts dorsally until finally (Ochotona) it almost 

 usurps the position usually held by the postorbital process of the frontals. 

 ( )n the other hand, by the transverse growth of the postorbital ridge the 

 familiar conditions in Carnivores and primitive Ungulates is produced. 



Post-tijm panic process of the squamosal. Apparently indicated in 

 Cynodonts (p. 121), well defined in primitive Marsupials. Fused with the 

 mastoid in Erinaceus (p. 261). 



Paroccipital processes. Absent in Cynodonts, absent or feebly developed 

 in the most primitive insectivorous Marsupials and Placentals; backwardly 

 produced in Erinaceus and certain Creodonts (c. g., Sinopa agilis); prog- 

 ressively developing in many Carnivores and Ungulates. 



Mastoid portion of periotic. Very large in Monotremes, conspicuous 

 in Marsupials and primitive Placentals becoming reduced in size in Fissipede 

 Carnivores and Ungulates. In Ilyrax only the upper portion of the mastoid 

 is exposed. In Arsinoitherium it aj^pears to be covered entirely. 



Tympanic. The mammalian tympanic, acconling to Broom, has been 

 derived from a certain element preserved in the skull of Oudenodon, Dicyno- 

 don and other mammal-like reptiles (p. 121). The primitive position of the 

 ring-shaped tympanic was probably oblique to horizontal (cf. Monotremes 

 p. 151, MicrogaJc p. 246). As shown by the thorough researches of van 

 Kampen (1905) the tympanic frequently offers characters of importance in 

 ])hylogeny, e. g., in the case of the Menotyphlous Insectivores (p. 274), 

 Dermoptera (p. 317), Chiroptera, Lemuroidea (p. 326), Arctoid and 

 iEluroid Carnivora. 



Auditory Ossicles. The several possible homologies and modes of 

 derivation of these ossicles are discussed on pages 125-143. Some of the 

 phylogenetic results of Doran's important researches on the mammalian 

 ossicula (1879) have been mentioned above under the various oi'ders. It 

 seems possible that an extension and thorough restudy of Doran's admirable 

 work might lead to new evidence regarding certain theories of relationship. 

 The form of the ossicula should be considered carefully with reference to 

 function. How do aquatic, fossorial, arboi-eal and other habits affect the 

 ossicula? How far are they influenced by the form or condition of the 



