418 



of the basis cranii, and which sends forward two processes, including 

 a membranous interspace. The auditory capsules are enclosed within 

 prolongations of the sides of the basilar plate ; and just in front of 

 and below them, the root of each process of the basal plate gives off 

 a solid prolongation, which passes at first outwards and downwards, 

 and then bends upwards and forwards, to rejoin the anterior part of 

 the process of the basilar plate of its side. An inverted arch is thus 

 formed, and the space included between its crura and the sides of 

 the cranium, constitutes the floor of the orbit. 



The posterior eras of the arch is divided into two, more or less 

 distinct, pillars, the posterior of which supports the hyoidean arc ; 

 the mandibular arc appears to be absent. 



The apertures whereby the cranial nerves make their exit are 

 situated in the side- walls of the capsule, that for the vagus lying 

 immediately behind the auditory capsule, while that for the trigeminal 

 is immediately in front of the same organ. The olfactory nerves per- 

 forate the anterior walls of the cranial capsule ; the optic, its lateral 

 walls between them and the trigeminal. 



The skulls of the Elasmobranchii, again, appear at first to be some- 

 thing quite different from either of these. The cranium is here a 

 cartilaginous box, more or less incomplete and membranous above, 

 and presenting on each side posteriorly a transverse enlargement, in 

 which the auditory organ is contained ; while anteriorly it expands 

 into a broad plate, which on each side overhangs the olfactory sacs. 

 The notochord and the membranous space have disappeared, or 

 their traces only are visible in the base of the cranium, whose 

 walls are, as it were, crusted with a multitude of minute plates of 

 bone. 



In the Chimcerce the inferolateral walls of the cranium pass into 

 a cartilaginous arch-like plate which form the floor of the orbit, and 

 whose posterior part, as in the Marsipobranchii, gives attachment to 

 the hyoidean arch ; besides which, a mandibular cartilage is connected 

 with the condyloid surface developed from the crown of the arched 

 plate. 



In the Plagiostomes there is also an inverted suborbitar arch with 

 a mandibular cartilage and a hyoidean apparatus, but the structure 

 of the arch is different from what obtains in Chimcera. 



The outer wall of that portion of the cranium which lodges the 



