42 PEOF. G. B. HOWES AND ME. H. H. SWINNEETON ON THE 



fourth of its processes (ex. 4 ), which now lies in an interspace between the palatine and 

 maxilla (cf. fig. X., ex. 4 ) l . The subnasal sheet (sb.) 9 now chondrified, wherefore it may 

 be termed the subnasal cartilage, has entered into continuity by means of a delicate 

 cartilaginous bar with the prsenasal process (nc.") of the primitive olfactory capsule, 

 and by a less considerable bar with the extranasal cartilage (ex., fig. 8). It extends 

 inwards, partly underlying the vomers, and effects (as indicated in fig. 12, s.b.) an 

 infolding of the outer lip of the posterior nares. 



Stage T. At this stage the characters of the definitive chondrocranium are all 

 present, and, owing to the increase in size, we found it unnecessary in dealing with it 

 to resort to microscopic sections and reconstruction. Fig. 10, PI. IV., delineates from 

 the left side this chondrocranium, divested of all dermal elements except the para- 

 sphenoid (ps.), now co-ossified with the basisphenoid (bs.). The most conspicuous 

 changes to be observed are two, (i.) the backward extension of the fourth otosphen- 

 oidal process (0s. 4 ) and its fusion with the auditory capsule at the base of the supra- 

 occipital bridge, (ii.) the elongation of the ethmo-nasal region, which is a main factor 

 in the final realization of the adult proportions and shape of the skull. As the result 

 of the fusion above-named, the lateral chondrocranial wall is now in a condition of 

 uninterrupted continuity dorsally with the olfactory and auditory capsules, and the 

 passage transmitting the fifth cranial nerve now becomes a perforated fenestra. In 

 proportion, size, and shape, the fenestrae have undergone a considerable amount of change, 

 mainly associated with the cartilaginous extension of the interorbital septum (s.i.). By 

 the extension of the lateral cranial wall, the process of closing in of the roof has 

 advanced, but there is no change of importance as compared with the preceding stage. 



The nasal capsule has become modified by the fenestration (n.f.") of the extranasal 

 cartilage. 



The cartilage-bones (which appear at Stage R, but, owing to their then feeble 

 condition, are not represented in our accompanying figures of the models at that, 

 figs. 9 & 12, PI. III.) are seen assuming shape at S (figs. 4, 5, 6, & 7), but at T they 

 are in the condition most favourable for description. They are 9 in all 3 median and 

 3 paired and with the exception of the basisphenoid all are otoccipital. 



When the skull is examined from beneath (PI. IV. fig. 6) the basisphenoid (b.s.) and 

 basi-occipital (b.o.) are seen to arise within the anterior and posterior regions of the 

 post-pituitary floor. The former extends laterally into the cartilaginous basipterygoid 

 processes (bs 1 .) 2 , which at Stage T terminate externally in dilated extremities, for 

 articulation of the pterygoid bones ; and it also extends forwardly into the trabecular 



1 This is presumably the " processus maxillarius posterior " of Gaupp, which he aptly compares with the 

 " antorbital" or palatal bar of the Anura and Ranodon (1898, Berichte, p. 8). 



3 "We have found traces in transverse section at Stages E-S of the paired ossification asseited by Baur (Zool. 

 Anz. Bd. xii. p. 45) to hold good for the basisphenoid and stated by Parker to be present in the Lacertilia 

 (' Morphology of Skull,' 1877, p. 216). 



