AXD IRREVERSIBLE EVOLUTION. 1053 



where it enters into the orbit the neo-orhital type and retain the 

 expression 2}(dceo-07-bitcd for the other. 



Among the Cotylosauria the Diadectidae (text-fig. 9 (5), Bia- 

 dectes), Pareinsauridjo, and Limnoscelidae show the same struc- 

 ture as the primitive Stegocephalians, the Captorliinimorpha and 

 the Procoloi:»honida3 tlie other. Pi-ocolojyhon (text-fig. 9 \(S)) is a 

 very agile Ootylosaurian, showing also many other signs of 

 specialisation : for example, a small lacrymal bone. In the 

 Captorhinimorpha, on the other hand, the limbs are specialised 

 to a rather high degree. 



Among the Testudinata that are somewhat allied to the Cotylo- 

 saurians, the relation of the frontal to the orbit varies. In 

 some primitive Testudinata, such as Triassochelys, Ghisternon, 

 and Kcdlokibotiuvi, the palaeo-orbital type is preserved : in some 

 other Amphichelydfe ah^eady the neo-orbital type occurs. 

 Curiously enough, the palfeo-orbital type occui-s also in the Pro- 

 tosteginaj and the Dermochelydae (text-fig. 9 (8)), while the 

 Lytolomidje and the Chelonidje (text-fig. 9 (7), Toxochehjs) show 

 the ueo-orbital type. In primitive Chclonidrc {Toxochehjs) and 

 in the embryos of Chelone (14) tbe neo-orbital type is more 

 marked than in the adult Chelone. In the rest of the Tortoises 

 generally the neo-orbital type is met with ; the palseo-orbital 

 type occurs, however, in the Platysterninas and Chelydridse, and 

 sporadically among the Emydida;. 



In Platy sternum, Dermochelys, and some Chelydridse, as 

 Macroclemmys, the palaeo-orbital type is associated with a 

 secondary enlargement of the bones forming the roof of the 

 skull and with the loss of the power of hiding the head under 

 the shield. In accordance with this, in Dermochelys, Chelone, 

 and Chelydra, the po^erior excavation of the parietal and the 

 squamosal is moi-e marked in the embryos than in the adult (14). 



Considering that in the most primitive Tortoises the capacity 

 of withdrawing the head had not yet been n.cquired, nnd that it 

 was but secondarily lost in Dermochelys, Chelone, Plaiystemum, 

 and Macroclemmys, it becomes evident that this feature and the 

 palajo -orbital type are connected with each other. Evidently 

 the palsBO- orbital type of the more specialised Tortoises has been 

 developed from the neo-orbital type, for the ontogenetical changes 

 observable in the ChelonidsB point in this direction. 



Among the Theromorpha the neo-orbital type dominates 

 Jflcrogomphodon (text-fig. 9 (9)). The palseo-orbitnl type is only 

 met with in the Cynognathidae {Protacmon, text-fig. 9 (10)). 

 In consequence of this it must be assumed that either the Cyno- 

 gua,thidjie retained a very ancient structure, or that also in this 

 case a reversal took place. Since in all Pelycosanrians that are 

 ancestral to the Theromorpha the neo-orbital type likewise occurs, 

 evidently the latter has to be assumed. As Pelycosaurians, it is 

 quite enough to mention the genera Varanosaurus, Sphenacodon, 

 Theropleura, and Dimetrodon, 



Among the Placodontidae, pala?o-orbital genera as Placodus 



