Dermal Bones of the Skull 



77 



an exoskeletal armour, formed by osseous plates, which 

 occur also on the roof of the skull, either under the form of 

 a thick and prominent ,,crusta calcarea", or under that of a mosaic 

 of more or less detachable bone-plätes ; the outlines of each ,,incrus- 

 ted surface", or of each bone-plate, correspond to those of a pilear 

 shield; the dorsal surface of the Orbits is co vere d with a lamina 

 supr aciliar is. Such forms are Ophisaurus, Trachysaurus, 

 Tiliqua, &c. I propose to designate this Type as the tectorbital. 

 These two main Types are those to which all other more or less 

 complicated exoskelelal formations, occurring in the Lacertilia, 

 are to be retraced. 



From the phylogenetic al point ofviewthe nudorbital type is 

 the more ancient, whilst true representatives of the tectorbital are, 

 up to now, only known from the palaeogene, and so will pro- 

 bably have existed at least as soon as at the End of the Mesozoic. 



With respect to the morphological conditions in which we 

 are interested at present, the tectorbital Type is by far the more 

 important. For its characterization let us choose the Anguinid 

 Genus Ophisaurus and the Scincid Genus Trachysaurus as exam- 

 ples. The body of these animals is provided with an armour of 

 osseous plates — constituting the exoskeleton — the skull pre.- 

 senting, on its surface, convex bony Clements, corresponding 

 to the pilear shield s. In consequence of this osseous scutellation 

 the shape of and the limit between the Single ,,membrane" bones, 

 i. e. primary dermal bones of the skull cannot be seen. The tem- 

 poral, gular and loreal region and in Trachysaurus the m.andibles 

 are covered with osseous plates, which correspond to the lepidotical 

 Clements. If we examine these exoskeletal bones on the skull of 

 Trachysaurus we will find that some of them are rather easy to be 

 detached from the endoskeleton (i. e. from the subjacent primary 

 dermal bones), so on the lateral regions of the parietal, on the 

 postfrontals and on the mandibles. The anterior lower part 

 of the maxillaries, the lower part of the praemaxillary and the 

 immediate area around the outer nareal openings correspon- 

 ding to the nasal shields is devoid of osseous plates. On some parts, 

 however, i. e. on the frontal bone and on the anterior median part 

 of the parietal bone (corresponding to the interparietal shield), as 

 well as on the region corresponding to the frontonasal and praefrontal 

 shields, the dermal armour adheres very strongly to the subjacent pri- 

 mary dermal bones (,,membrane" bones), in away as to occur rather 

 under the form of an ,,incrustation" than under that of separable 

 Clements. In some special cases, however, they might be isolated, as 

 in the nasal region of Ophisaurus for instance. Baroness A. M. 

 de Fejerväry-Längh, Ph. D., has shown in her Monograph of 

 fossil Ophisaurs, 1^) which is about to be published, that the 

 OS nasale of Ophisaurus can only be examined, with respect to 

 its shape, if the large and dilated exoskeletal plate, adhering to its 



Beiträge zu einer Monographie der fossilen Ophisaurier. 



7. Heft 



