1048 BARON FRANCIS NOPCSA ON REVKUSIIJLK 



foramen on esich side between each pubis and ischium (text- 

 fig. 8 (2)). Through this foramen the obturator nerve passes. 

 As these openings grow larger tlie central pelvic elements are 

 more or less reduced to rod-like bones. This change is analogous 

 to the one that occurs in the skulls of dillerent groups of reptiles, 

 for also in these the originally plate-like skull bones are reduced, 

 where they do not cover the brain-case, to rods that correspond 

 to the different lines of stress and strain. 



The tendency to develop more or less rod-like -ventral pelvic 

 elements is fairly well indicated in most tortoises (text-fig, 8 (3)), 

 with the exception of the marine ones, for the median ossification 

 is less marked iu tiie modern toi'toises than in the Amphichelyda^ 

 The same structure is also observable in the primitive Sauroptery- 

 gians {^N eusticosaurtis (text-fig. 8 (5)), Anaroscacrus). 



Among the Parapsida the rod-like pelvis is still missing in 

 Areoscelis but clearlj' indicated iu Pleurosuuras and well developed 

 in all 8(i[uamata. Among the Rhynchocephaliaus large ventral 

 pelvic openings are absent in Uoivesia and the llhynchosaurains, 

 but tliey are Avell developed in all other llliynchocephalians 

 (text-fig. 8 (7)) with exception of the Proganosauria. In all 

 Thecodontia, all Dinosaurs, and all Crocodiles the pelvic apertures 

 are always large. 



In contrast with this more or less plate-like ventral pelvic 

 elements are to be found in the specialised Sauropterygians 

 (text-fig. 8 (G)), in the Progajiosauria (text-fig. 8 (8)), and in the 

 Pterosaurians. Among the latter this feature is very noteworthy, 

 for it is especially well developed in the Pteranodontida;, which 

 are the most specialised members of the Order (text-fig. 8 (9)). 



JJermocheli/s, which is derived from some unknown chelonian 

 tortoise, has much smaller foramina obturatoria than all the 

 Ohelonidai, and retains in the pelvis a great amount of cartilage 

 throughout life (text-fig. 8 (4)). In this respect the pelvis of an 

 adult Dennochelys recalls somewhat the pelvis of llaUeria in an 

 early stage of development (11, 14). The resemblance which 

 Baur (1) detected between the pelvis of some Testudinata and 

 the pelvis of the Rhynchocephalia is, of course, only due to a case 

 of convergence, for the situation of the foramen obtui-atorium is 

 diflerent in the two groups. 



Comparing now the relationships of the reptiles mentioned in 

 the above lines, it becomes clear that in three cases plate-shaped 

 pelvic elements must have arisen from rod-shaped bones. The 

 Plesiosaurians m\ist have arisen from Nothosauiian reptiles, the 

 Proganosauria from Jurassic Rhynchocephalians, and the Pteran- 

 odontida) from tiiassic Thecodontia. T'hus these three cases are 

 quite characterii-tic cases of reversal. An indication of the same 

 soit of reversal is aftbrded by the differences that separate 

 Derniochehjs from the Ohelonida?. These differences show more 

 cleaily than the retuin of the postorbital bar in what manner 

 such a reversal begins. As suggested in the former case, it 

 starts by the retention of an embryonic stage throughout life. 



