152 THE ANCESTRY OF VERTEBRATfiS 



general we see in Elasmobranchs rather a tendency of the 

 skull to decrease than to increase in length, the highest 

 number both of epibranchial and of post-branchial cephalic 

 segments being found in the more primitive forms 

 (Notidanids). FuRBRlNGER was also wrong in assuming that 

 the skull in Amphibians is equal in length to that of 

 Selachians and that here also the occipital ventral nerve, 

 sometimes observed in early stages, must be termed z. It is 

 not z but X, the skull being shorter than that of Selachians. 



Head of Amniotes. — In sharks one (Scyllium, Pristiurus) 

 or two (Acanthias) post-branchial somites belong to the 

 region of the skull and a corresponding number of ventral occi- 

 pital nerves are found to leave the latter and to participate in 

 the innervation of the hypobranchial musculature. In Amniotes 

 it is generally stated that the hypoglossus consists of three 

 occipital ("occipito-spinal" after FuRBRlNGER) ventral roots, 

 that is one more than in Acanthias. Since in Amniotes the 

 number of gill-slits is one less, we reach the conclusion that 

 the backward extension of the Amniote skull corresponds 

 to that of Acanthias and that, as in the latter form, the skull 

 comprises eight segments. The number also of myotomes 

 observed in the occipital region of Amniote embryos' cor- 

 responds on the whole to that found in Selachian embryos. 



The hypobranchial or tongue-musculature is formed again 

 from the post-branchial myotomes, four in number, of which 

 one does not belong to the skull. The tongue musculature, 

 in fact, is supplied by a hypoglossus with three occipital 

 roots uniting with the first free ventral root to a plexus 

 which, however, in this case does not fuse with, the plexus 

 brachialis which in Amniotes often moves backwards a 

 fair distance from the head 



Thus the Amniote skull represents a neocranium, corres- 

 ponding in length to that of the Selachians. The intracranial 

 (occipital) hypoglossus-roots may be designated with the 

 same letters as those in Selachians, i.e. with the last, and 

 not the first, letters of the alphabet. Only if, with FuR- 

 BRlNGER (1897, p. 362), we call the last occipital nerve of 

 Acanthias a have we probably to do the same in Amniotes. 

 The "ganglion hypoglossi" discovered by FRORlEP (1882) 

 in the sheep and known also as FRORlEP's ganglion, being 

 the dorsal ganglion of the last head segment, evidently 

 corresponds to the dorsal ganglion found in the last segment 

 of the head in Acanthias (cf. fig. 31, sp. 8). In both cases it is 



