Dermal Bones of the Skull 



73 



As regards the crusta calcarea, it always occurs under 

 the form of most differently shaped, though well defined 

 ,,sculptures", offen presenting small pits, which are the traces 

 of the ,,Bindegewebskörper",^) or small spinous rugosities. 



Thus we must take care not to confound the ,,struc- 

 tural" sculptures of the primary dermal bones with the crusta 

 calcarea, representing the primitive stage of the secondary exo- 

 skeletal ossifications and formed by the corial lime deposits. The 

 crusta calcarea constitutes, in its very first stage, a complex 

 of rather independent lime concretions, coossifying later on with 

 the subjacent bones of the skull. For those who are in the lucky 

 Position to dispose over a large material it will not be too hard a 

 task to establish in which forms a true crusta calcarea occurs, 

 definitively elucidating hereby the origin and evolution of the 

 two different kinds of ,, sculptures", so unperfectly known as 

 yet and although so offen referred to in the various descriptions. 



In the Coecilians, some of which are provided with a 

 ,,degenerated" exoskeleton, hidden under the smooth epiderme, 

 and which are asserted to derive from Stegocephalian prede- 

 cessors, the skull seems to be devoid of secondary exoskeletal 

 Clements. 



The Urodeles or Tailed Batrachians are devoid of 

 conspicuous secondary exoskeletal Clements, the dermal bones 

 occurring on their skull deriving from a primary exoskeleton. 



The same thing occurs with almost all the AnuraorTailless 

 Batrachians. There are, nevertheless, some interesting ex- 

 ceptions, very instructive from the Standpoint of the development 

 of the exoskeleton in general. With respect to this subject Mr. 

 G. A. Boulenger^ö) writes as follows: ,,0n trouve une plus ou 

 moins grande quantite de substance calcaire dans la peau du Cra- 

 paud commun; ces depots calcaires peuvent etre tres developpes 

 dans la peau du dos de certaines especes des genres Megalopkrys, 

 Nototrema, Phyllomedusa et Lepidobatrachus] d'autres Anoures 

 possedent un bouclier dorsal osseux, libre {Ceratophrys) cu ankylose 

 aux vertebres (Brachycephalus) ." These dermal ossifications consti- 

 tute a secondary exoskeleton — just like the osseous plates 

 occurring in Ostracion — in Opposition to the ancestral primary 

 exoskeleton, from which the socalled ,,membrane bones" of the 

 skull derived. Thus it is advisable to distinguish within the dermal 

 bones of the skull, in general, two phylogenetically different kinds, 

 i. e. primary and secondary dermal bones. Such secondary 

 dermal bones are also present on the skull of a European Genus, 

 viz. on that of Pelobates.^^) The secondary dermal bones are in- 



^) Cfr. Leydig, op. cit. p. 74. 



10) Les Batraciens &c.,Encycl. Sc, Bibl. de Zool., Paris, 1910, p. 18— 19. 

 Cfr. Gadow, in: The Evol. of Horns and Antlers, Proc. Z. Soc. 

 London, 1902,. p. 208: ,,In Pelohates the skin of the upper surface of the 

 head is partly 'co-ossified with the underlying cranial bones, giving them a 

 pitted" ( ? rather spinous!) „appearance. Now, frontal and parietal being 



7. Heft 



