ON THE VERTEBRATE SKELETON. 191 



petrosal (i6), and the retention by a great proportion of this capsule of the 

 acoustic labyrinth of its primitive cartilaginous state, it occupies a smaller 

 interval between the alisphenoid (r>) and exoccipital (2). It no longer pro- 

 trudes as a large bony wedge (as in figs. 6 and 7> 10) into the cranial cavity, 

 but permits the alisjDhenoid to come into connection with the exoccipital. 

 The result of this further retrogradation of the alisphenoid, in regard to the 

 relative position of the outlet of the third division of the fifth, is analogous 

 to that which occurs in the sheep. We saw in that mammal, through the 

 recession of the squamosal, the foramen ovale advanced from the posterior to 

 the middle part of the alisphenoid ; in the crocodile, through the further re- 

 moval from the cranial cavity of the interposed petrosal, the foramen ovale is 

 advanced to the anterior border of the alisphenoid ; which border, in fact, it 

 notches, the nerve escaping by a common foramen or ' trou du conjugaison' 

 between the alisphenoid and the orbitosphenoid, the hole, however, being 

 principally formed by the alisphenoid (fig. 9, tr). This position of the ' fora- 

 men ovale' loses all its value as an argument in favour of the petrosal cha- 

 racter of no. 6, by analogy with the position of the foramen ovale in man 

 or the ape, when we take into consideration the necessary consequences of 

 the successive withdrawal of the squamosal and true petrosal from the inner 

 surface of the cranium in descending to the reptiles. The orbitosphenoid 

 (fig. 9, 10), notwithstanding its great relative size, retains all its essential cha- 

 racters: it is perforated or notched for the exit of the optic nerves (op) and 

 first division of the fifth pair (s); it rests upon the presphenoid (9) below, 

 and likewise, through its backward development, partly upon the basisphe- 

 noid, and it articulates with the frontal (11) above, and also through the 

 same backward extension with the parietal (7) ; it constitutes the anterior 

 border of the lateral bony parietes of the cranium, which are interrupted 

 by the orbits, and separated by their interposition in saurians and fishes 

 from the rhinencephalic part of the cranial cavity (at 14, fig. 9). The cha- 

 racters, in fact, of the orbitosphenoid are so clearly manifested in the cro- 

 codile, that Cuvier, having been led by the increased share, as compared 

 with mammals, which the crocodile's alisphenoid (fig. 9, 6) takes in the form- 

 ation of the otocrane, to regard it as the petrosal, and yet perceiving the 

 essential characters of the orbitosphenoid in the bone (ib. 10) anterior to it, 

 was driven to the conclusion that that bone represented both orbitosphe- 

 noid ('aile orbitaire du sphenoide') and alisphenoid (aile temporale du sphe- 

 noide). The cold-blooded crocodile, however, is not exactly the animal in 

 which we should expect to find so unusual an instance of obliteration of 

 sutures, as that between the alisphenoid and orbitosphenoid*. The actual 

 and most characteristic modification of the orbitosphenoid in the crocodile's 

 skull, is its retrogradation together with the alisphenoid, or rather the main- 

 tenance of its normal connection therewith by increased antero-posterior 

 development, whereby it comes into communication above with the parietal 

 (7) and below with the basisphenoid (5) ; whilst the alisphenoid, in like 

 manner, gains a connection with the supra-occipital (3) above and the basi- 

 occipital (1) below ; although it still retains its more normal relations with the 

 parietal, and rests in great part on the basisphenoid (5), as the orbitosphe- 

 noid rests in great part upon the pre-sphenoid (9.) The superior connec- 



* No one better appreciated the characteristic persistence of the sutures in the crocodile 

 than Cuvier, when his attention was not diverted from it by a favourite hypothesis. "Le 

 crocodile a cela d'avantageux a l'etude de son osteologie, que ses sutures ne s'effacent point, 

 du moins n'en a-t-il disparu aucune dans nos plus vieilles tetes," is the remark with which 

 he commences his article on the determination of the bones of the head of that reptile 

 (Ossemens Fossiles, 4to. v. pt. ii. p. 69) : but at p. 76, a suture is assumed to be effaced, 

 which is present in most mammals and all cold-blooded vertebrates, where a wider space 

 does not intervene between the alisphenoid and orbitosphenoid. 



