AND iiiup:vei?,sibl]!; evolution. 1055 



saurians the fiontnl sliows a decided tendency to vanish alto- 

 gether, and, besides tliis, in the more specialised long-snouted 

 Plesiosanrians (the Piiosaurians) the tendenc}' exists to develop 

 large supraorbital bones. This tendency is well observable in 

 the genera. Feloneustes, BrachyaucheniaH, n\n\ Trinacromeruvi. 

 In these genera the pi'efrontals and postfrontals are long and 

 narrow bones. On account of the reduction of the frontal these 

 animals revert at first to the pala3o-orbital type, but when the 

 broadening of tlie head sets in they develop on other lines. 



In the Parnpsida, that include the Areoscelin, the Acrosauria, 

 and the Squamata, the frontal nearly always separates the pre- 

 frontal and the postfrontal. While it borders the orbit in Areo- 

 scelis, Pletirosaurus, all primitive Chameleons (18) and many 

 Lacertilians {Plaiecarpvs, text-(ig. 9 (12)), it is excluded in some 

 Lacertilians from the orbit by a supraoi-bital bone (text-fig. i) 

 (13), Varanus). //eZoc(fe?-?»a (text-fig. 9 (11)) and the specialised 

 Chameleons (17) differ from all the other Squamata in showing 

 the palajo-orbital structure, but this may be due to a leversal. 

 Thus in this group the structui-e varies. 



In the Diaptosaurians (text-fig. 9 (U5), JiJvparkeria), the Dino- 

 saurs, and the Crocodiles, the prefrontals and the postfi'ontals 

 never meet. In some Crocodiles however, and in the ortho- 

 podous Dinosaurs supraorbital bones are developed (text-fig. 9 

 (17), Gamptosaumis). 



For the neo-orbital type, in which .a supraorbital bone is 

 present, Fejcrvary's term, tectorhitcd (7), can be adopted. Since 

 the supraorbital bone is only developed in few groups of reptiles, 

 the tectorbital type is evidently new. 



Proceeding now to group the primarily palfco-orhital, the neo- 

 orbital, the secondarily pala30-orbita], and the tectorbital types 

 accoi'dingto chronological order, it is soon seen that the primarily 

 palteo-orhital types are either permian reptiles or such that are 

 closely allied to permian reptiles. The neo-orbital type occurs 

 in different groups from the Permian upwards, it is most marked 

 in the most advanced reptiles ; the secondarily palfeo-orbital forms 

 are found from the Tiias upwards, but mostly among compara- 

 tively low postcretaceous reptiles; finally tectorbital types occur 

 exclusively from the Jurassic upwards. 



The average conclusion to be drawn is that in primitive 

 reptiles (Cotylosaxiria, Tortoises) a reversal could easily occur from 

 the neo-orbit.al type to the palaeo-orbital type; that, however, in 

 the more highly developed reptiles (Crocodiles, Dinosaurs) the 

 broadening of the skull could no longer be attained by a reversal 

 but only by the development of a new bony element. Compara- 

 tively primitive reptiles, as Sauropterygia and Squamata, seem 

 to be intermediate between the two extremes. The primitive 

 nature of the Squamata is best shown by the circulatory and 

 respiratory organs. 



In two most important papers Weidenreich (16, 17) pointed 

 out that in some living animals characters occur that are very 



