Dermal Bonrs of the Sknll 



71 



the skull seem to be throughout the off spring of a primary 

 exoskeleton. There are, however, representatives of the Class in which 

 the presence of secondary exoskeletal elements (bone-plates) 

 can be established. 



Such secondary exoskeletal plates are present on the 

 skull of Ostracion for instance, and the bony gular plates of the 

 ancestral representants of the Dipnetisti may probably also be 

 referred to this kind of elements. The praedental bone, occurring 

 in some Fishes (Onychodontidae, Aspidorhynchidae), referred to 

 by Prof. Abel as constituting a ,, sekundäre . . . Bildung"*) and 

 the praeethmoideum (present in Cyprinus for instance) are 

 probably also belonging to the secondary exoskeletal ossi- 

 fications. Future investigations, leading to a more exact knowledge 

 of the phylogenetical evolution of the single elements constituting 

 the Fish-skull, will surely enrich this series by many other 

 examples. 



With respect to the Fishes it is important to point out the 

 fact that in most of the cases in which the presence of secondary 

 exoskeletal elements could be established, the mentioned ossif ications 

 occur, on the skull, but imder the form of single ,,supernumerary" 

 bones, and the occurrence of numerous bony plates, united to a 

 coherent (secondary) exoskeletal armour overroofing the ,,mem- 

 brane" bones (like in Ostracion), must be considered as a rather 

 exceptional feature 



The most ancient representatives of the Orders Osteostraci^), 

 Antiar cht and Arthrodira present a highly developed exo- 

 skeleton. The structure of the exoskeletal elements of the 

 Anaspida, which are the oldest Vertebrates we know of, could not 

 be established, though it is probable that their dorsal (?) crest^) 

 might be looked upon as containing dermal ossif ications. The ele- 

 ments of the exoskeleton of the Osteostraci and Antiar chi cannot be 

 homologized with the dermal bones of the skull in any other Verte- 

 brate. There is no proof as yet of the exoskeleton of these two 

 Orders being a secondary one*- — like in Ostracion — , and that, 

 below it, the presence of an ossified skeleton, containing homologi- 

 zable primary dermal bones (i. e. so called ,,membrane" bones) 

 ought to be presumed. It is, at least for the present, more reaso- 

 nable to consider these two groups as constituting extinct side- 

 branches, the endoskeleton of which was probably merely 

 cartilaginous, whilst their exoskeletal elements may presu- 



*) O. Abel, D. Stämme d. Wirbeltiere, Berlin ii. Leipzig, 1919, p. 54—55. 



^) Including the „Heterostraci'\ (Cf. O. Abel, op. cit. p. 71.) 



*) Mr. Traquair and other palaeontologists consider this crest as 

 ventral, whilst Jaekel, and reeently Freiherr Stromer v. Reichenbach, 

 suggested it to be dorsal. Such an orientation of the body would make 

 the general topography of its morphological stractures decidedly more 

 comprehensible. In this case the tail would represent, of course, the hypo- 

 batic type, instead of the epibatic by which all palaeozoic Fishes, knownup 

 to now, are charaeterized. 



7. Heft 



