424 Bulletin American Museum of Natural Hi.stunj. [\"ol. XX"\'II, 



upward to meet frontals — perhaps correlated with backward growth of 

 rootless incisors. Internal horizontal process homologous with the diunb- 

 bell bone of Ornitliorhynchus. 



MaxiHo'. Overlapping premaxilliie and nasals; in contact internally 

 with vertical ])late of palatine; overlapped by palatines, jugals and lachry- 

 mals, finally gaining broad contact with frontals. 



Palatines. Vertical plate pierced or notched by the sphenopalatine 

 foramen. Vertical plate more or less extensive in Marsupials, Carnivores, 

 Rodents, Primates, Ungulates and Edentates, reduced in Insectivores. 



Palatal vacuities. In the Marsupials may 'be j^artly secondary, partly 

 a result of the primitively imperfect development of the secondary palatal 

 plates of the maxillaries and ])alatines. In the Insectivora palatal vacuities 

 may well be secondary (c/. pp. 220, 286). 



Pterygoids. Derived from pterygoids of Cynodonts, possibly in the 

 manner suggested on page 120. Primitively consisting of thin lamelh^, 

 articulating with basisphenoid and posterior extension of vertical plate of 

 palatines, and closely appressed to pterygoid wing of alisphenoids. Often 

 becoming reduced to a thin strip (e. (/., Equus). 



Pterygoid fossa'. Formed by tin' insinking of the ectopterygoid muscles 

 which are originally (e. g., Microgale, Arclomys) fastened on top of the com- 

 bined pterygo-alisphenoid ridge but gradually separate this ridge into its 

 two coziiponents, so that the (ecto) pterygoid fossae become bounded internally 

 by the pterygoids (which are sometimes produced into a long hamular proc- 

 ess), externally by the pterygoid wing of the alisphenoid {e. g., Erinaceus, 

 Lepus, Canis, Homo). 



Nasals. Distal end originally terminal and pointed. Proximal end 

 spreading widely (e. g., Didelphis, Meniscotherium, Mesony.r) and almost 

 excluding the maxillary from contact with the frontals. By the upgrowth 

 of the maxillary the proximal expansion of the nasals becomes overlapped 

 so that finally only the internal mesial fork of the proximal end remains 

 exposed to view (e. g., Erinaceus, Canis, Lemur). Sometimes the external 

 proximal portion of the nasals is overgrown by a forward process of the 

 frontals (e. g., Ursus). 



Lachrymals. Overlapping maxillaries, frontals and vertical plate of 

 palatines, articulating with malars, usually pierced by lachrymal duct. 

 Absent in Monotremes. Orbital portion large in Marsupials, with marginal 

 foramen. Lachrymal becoming extended anteriorly with the prolongation 

 of the face in Creodonts and certain Ungulates. In Insectivores becoming 

 reduced, dorsally displaced and overlapped by alveolar portion of maxillary. 



Frontals. Originally very small and anteriorly placed (Monotremes), 

 covering chiefly the ethmoids and rhinencephalon; always forming dorsal 



