Dermal Bon es of tho Skull 



101 



9^ In the Fishes the primary dermal bones, though practi- 

 cally f orming a part of the osseous endoskeleton, preserved 

 their ancient exoskeletal charakter to a higher degree than in 

 the other Vertebrates. The secondary dermal ossifica- 

 tions of the skull generally occur, in this Gass, imder the 

 form of rather single elements; in some cases [Ostracion) they 

 may, however, form a coherent armour, overroofing the pri- 

 mary dermal bones. 



10® The exoskeletal plates occurring on the head of the Placo- 

 dermi are probably also primary dermal bones — and not 

 secondary as in Ostracion — , but they are not homologizable 

 with the primary dermal bones of the coinocranian Fishes. 



11® The ,,sculpture" of the cranial bones in the Batrachians and 

 Reptiles may be retraced to the presence ofacrustacalcarea, 

 or it may represent a structural particularity of the 

 respective cranial bones themselves. 



12® The structural sculpture is radial or concentric, or rather 

 both. It may occur on both the primary and secondary 

 (and also on the dermal bones tertiary ones). 



13® The crusta calcarea — which is formed by the corium — 

 appears as a pitted or spinous incrustation, constituting, 

 in its very orimental phase, a complex of rather independent 

 lime-concretions, which coossify later on with the subjacent 

 bones of the skull. 



14® The crusta calcarea of the skull is morphologically homo- 

 logous with the whole of any secondary dermal bone- 

 plate of the body. 



15® The crusta calcarea of the Batrachian and Reptile 

 skull may represent ei t her the first stage of the formation of 

 secondary exoskeletal ossifications, or the remnants (rudi- 

 ments) of degenerated secondary exoskeletal elements. 



16® In the recent Anura some forms are provided with bony 

 ,,stegar' elements, i. e. secondary dermal ossifications; 

 these do not represent ancestral features, inherited from some 

 Stegocecephalian predecessors, but they are later acquisi- 

 tions of the Anuran Stem. 



17® Among the Lacertilians two main Types could be distin- 

 guished with respect to the dermal bones of the skull: the 

 nudorbital Type, in which a supraciliar y lamina is absent, 

 and in which the skull is generally devoid of secondary 

 dermal ossifications, and the tectorbital, in which a lamina 

 supraciliaris and other secondary dermal ossifications are 

 present. 



18® The lamina supraciliaris is formed by the fusion of small 

 polygonous plates, a fact especially well expressed in the 

 young specimens of some more ancestral representatives of 

 the tectorbital Type. 



7. Heft 



