Dermal Bones of the Sknll 



91 



sicles present on the carapacc of Toxochclys are also tertiary 

 exoskeletal Clements. '^^) 



In Mammalians two kinds of exoskeletal elements are, as 

 a rule, distingiiished : the horns - — which are looked upon by 

 most authors as elements originally independent from the frontal 

 bone, i. e., using oiir present terminology, constituting true Se- 

 eon dar y dermal bones, — and the dermal bone-plates. Not 

 all horn-hke features are, however, diie to ossifications. It is a well 

 known fact that the ,,horn" of the Rhinocerotidae is purely corneous, 

 and does not contain any osseoiis ,,axis". This, corneous horn is 

 siipported b}' a slight, hyperostotic elevation of the nasal bones, 

 faintly projecting into its hollowed base. Thus the osseoiis sub- 

 stance of the low basal ,,stump" apparently belongs to the actual 

 endoskeleton, i. e. to primary dermal bones (the nasals). In 

 Opposition to this the horns and antlers of all other Mammals 

 contain or purely consist of an osseous material. The Cavicornia 

 possess an osseous cone, protruding from the frontal bone, 

 which is covered by a horny substance produced by the epiderm. 

 The mentioned cone is proved to originate in the Cavicornia from a 

 separate corial i. e. dermal bone, its centre of ossification 

 being originally independent from the frontal bone. This 

 secondary dermal bone, the os cornu, fuses already in early 

 ontogenetical stages, i. e. in the juvenile individual, with the 

 antagonisticprotuberances sent off by the f rontal (= frontal 

 apophyses). This palingenetic mode of development throws 

 light upon the relativelyrecent corial origin of this element, i. e. on 

 its phylogenetical independency from the primary dermal bones. 



As far as I am acquainted with the mammalogical literature, 

 the ontogenetical development of the simple (monostyle) or mon- 

 axone osseous cone of the bifurcated and thus antlers-like horns 

 of Antilocapra has not as yet been established. Prof. iSitsche^^) 

 pronounced the very plausible opinion that the Osteogenesis of the 

 oFseous horn-cones of this Genus will probably be the sam.e as 

 in other Cavicornia, and so the cranial element in question ought to 

 be, also in this case, looked upon as constituting a secondary 

 dermal bone (,,Cutisbildung und Epiphyse"), having, later on, 

 fused with the frontal. 



In the Girajiidae the three horns are also typical secondary 

 dermal bones. 



In the Cervidae the origin and homologization of the antlers 

 constitutes a ver}' difficult problem. Prof. Nitsche assures that 

 these bony appendages are not der mal bones". Using our present 

 terminology, this would mean that the antlers do not represent 



^1) Cfr. O. Abel, Gnmdz. d. Paläobiol. d. ^Yirbelt., Stuttgart, 1912, 

 p. 611 — 614. For Toxochelys see Abel, Stämme d. Wirbelt., p. 395. 



Studien über Hirsehe (Gattung Cervus im weitesten Sinne), Heft I, 

 Untei-s. üb. mehi^tangige Geweihe u. d. Morph, d. Hufthierhörner i. allgem., 

 Leipzig, 1898, p. 78. 



") Op. cit. p. 68 & Textfig. 8. 



7. Heft 



