1014 



the branchial basket it is even united with the plexus brachialis 

 into a common stem, the j)lexus cervico-brachialis, which only, 

 distally splits into a branch to the hypobranchial musculature and 

 one to the pectoral fin (fig. 2). This common plexus was seen by 

 Hoffmann (1901, p. 39) to form during development in exactly the 

 same way as is described by ISeal for the hypoglossns of Petro- 

 myzon. The skull in Acanthiris containing 8 segments, of which 6 

 post-otic, we may expect that of the four roots of the hypoglossus 

 or plexus cervicalis after Hoffmann's statements the anterior two 

 (5''' and 6^'' post-otic somite) will pass through the skull. This 

 indeed proves to be the case (Fürbringf.r, 1897, p. 362). In no 

 Selachian is the number of intra-cranial (occipital) hypoglossus roots, 

 as far as has been definitely stated, more than 2 (Fürbringek, I.e. p. 

 404), often one or none. From this the approximate number 

 of post-branchial segments incorporated into the 

 skull may be derived. Provisionnally we must say "approx- 

 imate" since we have insufficient evidence as to the part ici|>at ion 

 or non-participation of the last epibranchial somite. Nkal (1897, 

 p. 461) su|)poses that a transitory crowding forward of the posterior 

 gill-slits causing the last one to lie under and not behind the last 

 epibranchial myotome, as observed by himself in Acanthias and by 

 Miss Platt (1897, p. 458) in Necturus, sometimes allows the ventral 

 giowth of a myotome, which otherwise would be prevented. 



In front of the occipital hypoglossus roots (y, z, of Fürbringek) a 

 few ventral occipital nerves may still be found which either remain 

 independent {Hexanchu.s, Heptanchus) or at first may join the plexus, 

 but sup[)lying only epibi-anchial musculature {x,iü,v, of Fürbringer), 

 and therefore are comparable to the ventral roots of 

 the f i 1- s t six post-otic myotomes of Petroinyzon f o r 

 which the same hold s. Thus of the occipital myotomes and 

 nerves only the anterior ones are primarily epibranchial, the one or 

 two posterior ones are of post-branchial origin, but have become 

 epibranchial only secondarily by the dispersion of the gill-slits. 



In He.vanchus and Heptanclms where the number of gill clefts is 

 greater, but the number of occipital hypoglossus roots not less than 

 in pentanch sharks, the skull accordingly may be expected to com- 

 prise at least one or two segments more than in Acanthias. This 

 is also confirmed by what follows. The sphere of influence of the 

 vagus on the dorsal roots following it in Scyllium etc. reaches to a 

 little behind the cranio-vertebral limit, the second free spinal 

 ganglion only is again nearly normal, the first as a rule being absent 

 (Fürbringer, I.e. p. 392). In Acanthias there is formed in the last 



