194 



VERTEBRATE SKELETON 



foramen. Other ossifications may join these paired bones. Many 

 lower mammals have a fold of membrane (tentorium) between cere- 

 brum and cerebellum, which may ossify and join either supraoccipital 

 or parietal, while Monotremes and dolphins have a similar ossification 

 in the membrane (falx) between the cerebral hemispheres, this 

 forming a ridge on the under side of the parietals, small in Echidna, 

 larger in the duckbill. Monotremes, some Edentates and Insecti- 

 vores have a 'pterotic' bone between squamosal and parietal which 

 may exclude the former from the cranial wall. It develops in some 



Fig. 204. — History of antler of deer (Nitsche, in Weber, '04). A, first appearance 

 of spike as an apophysis of frontal bone; B, skin retracted from spike, resorption sinus 

 appears at base of spike; C, loss of spike; D, E, stages in development of antler of second 

 year; covering of pedicel with skin; outgrowth of antler, continued in F, with axis 

 and tine shown, the whole covered with skin and hair (" velvet ') ; G, skin retracted from 

 antler, c, corium; e, epidermis with hair;/, frontal bone; s, spike. 



Other forms, but soon unites with the parietal. Its position suggests 

 a postfrontal. 



The frontals, paired in origin and remaining separate in most 

 mammals (uniting in Monotremes, Rhinoceros, Elephas, Insectivores 

 and Primates) foim the cranial roof between the orbits, meeting the 

 aUsphenoids laterally. Each sends a process into the wall of the 

 orbit and usually a postorbital process which meets the zygomatic, 

 forming the posterior border of the orbit. As there is a separate 

 centre in this process, it may be postfrontal, a supposition more 

 probable than that pterotic and postfrontal are homologous, since 



