Dermal BoDes of the Skull 



99 



external irritation, the effect of this latter reaching the 

 osseous cranial surface at last. And so it is clear that the 

 cranial bones were only indirectly attained by the effect of the 

 (external) irritation which acted upon the complexion, and it 

 is very probable that their apophytic growth has been a mere 

 consequence of an other, immediate irritation caused by bud- 

 ding ossicles (the ossa cornuum) formed by and lying in the 

 derm. 



The evolution of the horns of other cornigerous Mammalians, 

 [Sivatheriidae [Order Artiodadylä], Dinoceradidae [O. Amblypoda], 

 Arsinotheriidae [O. Emhrithopodd], Titanotheriidae [O. Pertsso- 

 dactylä] and Mylaganlinae (Family: Castoridae, Order Rodentia]), 

 and thus the homologization of their elements, could only be cleared 

 Up by future special investigations. 



Finally the horns of the Dasypodid Genus Peltephilus 

 (O. Xenarthra) shall be mentioned, which seem to be the horn-like 

 modifications of the foremost plates of the secondary exoskeletal 

 armour overroofing the upper parts of the skull. 



Let US see now the dermal bone- plates of the Mammalia. 

 Such are present in the fossil Xenarthran Mylodon (family 

 Megatheriidae), the corium of which contained numerous dermal 

 bone-plat es, representing the secondaryexoskeleton. A power- 

 ful secondary exoskeleton is present in the Xenarthran 

 Glyptodontidae, consisting of sculptured polygonous dermal bone- 

 plates, which acquire a rather spinous aspect on the tail. The 

 he ad is covered with a cap-like mosaic of exoskeletal elements 

 (cfr. Glyptodon, Doedicurus) ; these elements are, in the same way 

 as on the body, quite independent from the primary dermal 

 bones of the skull, i. e. they are not ankylosed to them. The same 

 phenomenon occurs in the recent Dasypodidae (Order Xenarthra) 

 and Manidae (Order PhoUdota), in which the secondary dermal 

 bones of the head, covered by a horny epidermal layer, are not 

 connected by any osseous tissue to the primary dermal bones 

 of the cranium. It is only the above mentioned fossil Dasypodid 

 Genus Peltocephalus in which the secondary exoskeletal plates, 

 overroofing the upper surface of the skull and the Upper part of 

 the temporal region, seem to be more intimately attached to the 

 subjacent primary dermal bones. In all the Mammalians, however, 

 the primary dermal bones are clearly distinguishable from the 

 secondary exoskeletal plates, so that the homologization of these 

 elements is beyond all doubt. This state of things, due to the 

 relatively loose connexion existing between the secondary and 



We saw that in the horn of the Rhinocerotidae a bony distal element, 

 corresponding to the os cornu, seems to be absent; yet the question arises, 

 if this absence is primary and not secondary, due to a later suppression of 

 an osseous oriment of yore by the „hypertrophic" development of the 

 corneous layers constituting the horn. May be also that the special structure 

 of the integument of these ,,Pachydermes" prohibited, ab initio, the forma- 

 tion of an epiphytic dermal ossicle. 



7* 7. Heft 



