DEVELOPMENT OP THE SKELETON OF THE TTJATARA. 41 



Stage Q the front of the foramen for the fourth nerve has become much elongated 

 and brought into line with the second otosphenoidal process, so as to give the 

 appearance of a continuous bar. That this is not the case, however, is proved by 

 the fact that the second otosphenoidal process in reality is now represented by the 

 insignificant tract (fig. 8, os?) lying between the ventral extremity of the cranial fenestra 

 if-n.) above and the dorsal extremity of the optic fenestra (/.") below. Its apparent 

 forward continuation at this stage is due to the appearance in the cartilaginous inter- 

 orbital septum of yet another fenestra (/./., PL III. fig. 8), which is of necessity median. 

 Like the lateral cranial fenestra if.n.), it is wholly membranous, and it alone merits 

 the term interorhital fenestra. 



Turning to the occipital region, the foramen magnum is now seen to be completely 

 surrounded in cartilage, as the result of union between the upper ends of the occipital 

 cartilaginous lobes and the posterior walls of the auditory capsules, resulting {of. PI. III. 

 fig. 9, e.o.) in the enclosure of the afore-mentioned interspace transmitting the 

 cranial nerves ix. to xi. above, and its conversion into a foramen. With these changes 

 another, probably correlated with the afore-mentioned upheaval of the ethmosphenoidal 

 plates, is recognizable, viz. the appearance in the postero-dorsal cranial roof of a 

 spatulate cartilage {so.', PI. III. figs. 8 & 11), which in its general relationships recalls 

 that figured and described by Parker^ in Lacerta and Zootoca k% "supra-occipital." 

 This is seen to arise posteriorly from a massive tract of cartilage overlying the foramen 

 magnum, which appears to us more extensively derived by the overarching of the 

 auditory capsules than of the occipital lobes. When the chondrocranium is viewed 

 from the side, at this stage, this " occipital " cartilage is seen to be arched, and to 

 follow, with the antero-dorsal border of the expanded ethmosphenoidal plate, the arc of 

 the circle described by the top of the head, which at the earlier stage was followed by 

 the third otosphenoidal process {os?, PL III. fig. 5). In this respect, the two may be 

 said to have been correlatively modified in accommodation to the roofing-in of the brain, 

 now possible under the diminution in relative rapidity of the growth of the central 

 nervous system. 



An interesting detail at this stage is the presence in the basi-occipital region of a 

 minute depression, involving the lower border of the foramen magnum. Conversely, 

 the ventro-lateral border is on either side rounded and convex, the whole giving to the 

 chondrocranium, especially when viewed from above (PL III. fig. 11) or from beneath 

 (ficf. 12), the appearance (as has been pointed out by Gaupp ^for the embryo lacertilian) 

 of being doubly condylar. 



In the nasal region the extranasal cartilage (ex., PL III. fig. 8) has increased in 

 size, but it has undergone no conspicuous change, except for the elongation of the 



' Parker, W. K. : Phil. Trans, 1879, p. 595, 

 ^ Gaupp, A. : 1898, Beriohte, p. 6. 



VOL. XVI. — PART I. No. 6. — February, 1901. g 



