38 PROF.T. J. VARKER ON CARCHARODON RONDELETII. [Jail. 18, 



anterior and ventral (fig. 23, v^), former] of two distinct strands, and 

 a posterior and dorsaP (v^), which arises mainly from the dorsal 

 aspect of the metencephalon (fig. 20), immediately caudad of the 

 corpvis restiforme, but also receives a small bundle of fibres arising 

 from the lateral region of the metencephalon, in common veith the 

 root of the eighth {viii.). The facial (vii.) has one main root formed 

 of two strands, the ventralmost of which is intimately united with 

 the single root of the auditory nerve (viii.). 



Both dorsal and ventral roots of the fifth divide before leaving the 

 skull, so that the nerve passes through the trigeminal foramen in four 

 parts (fig. 20), each of which perforates separately the membrane of 

 the foramen. 



The sixth nerve (vi.) arises by three distinct roots, the posterior 

 of which is very slender and soon unites with the middle root. 



The vagus (x.) is an immense nerve arising by six lateral roots, of 

 which the first four and the last two unite to form separate bundles, 

 which leave the cranial cavity before joining into a common trunk. 

 On the right side (fig. 20) the posterior root is double, and its 

 hindmost factor arises at least 1 centim. caudad of the calamus 

 scriptorius. 



A short distance cephalad of the origin of the posterior root of the 

 vagus there arises from the ventral aspect of the metencephalon a 

 distinct though small root (x^), formed by the union of several 

 strands. This evidently corresponds with the nerve thus described by 

 Balfour ^: — " The main stem of the vagus at a short distance from 

 its central end receives a nerve which springs from the ventral side 

 of the medulla, on about a level with the most posterior of the true 

 roots of the vagus. This small nerve corresponds with the ventral 

 or anterior roots of the vagus described by Gegenbaur, Jackson, and 

 Clarke (though in the species investigated by the latter authors these 

 roots did not join the vagus, but the anterior spinal nerves). Similar 

 roots are also mentioned by Stannius, who found two of them in the 

 Elasmobranchs dissected by him ; it is possible that a second may 

 have been present in Scyllium, but have been overlooked by me, or 

 perhaps may have been exceptionally absent in the example 

 dissected." 



As the nerve-roots in Carcharodon were made out while the tough 

 pia mater was quite intact, I feel satisfied that no other ventral root 

 of the vagus was present in my specimen. From the direction taken 

 by the nerve it appears to join the vagus, not the spinal nerves ; but 

 it was unfortunately severed, as shown in fig. 22, when the brain was 

 removed. 



DESCRIPTION OF THE PLATES. 



Plate IV. 

 Fig. 1. Cranium of Carcharodon rondelefii, dorsal aspect, X j. 

 2. Cranium of Lamna cornubica, dorsal aspect, x 5. 



^ Tliis root propei-ly belongs to the seventh, as shown by Balfour and Marshall. 

 ' ' Elasmobranch Fishes,' p. 106 (Works, Memorial Edition, vol. i. p. 419). 



