418 



IIARKIS lIAWTIIOliXE WILDE1{ ON 



seen the internasal plate of the primordial skull. Viewed from the ventral side, the two 

 vomers may be said to overlap) the parabasal, and the areas of contact are clearly marked 

 upon the dorsal surface of the former bone and the ventral surface of the latter. The 

 frontal bone comes in contact with the vomers dorsall}', and the articular surface for this 

 forms a long, narrow ridge, running along the lateral border above the row of teeth. At 

 the extreme anterior end, the two vomers come in contact with each other, the symph3^sis 

 being marked by a few jagged, irregular projections. The postero-lateral border of the 

 palatine portion is recurved and exhibits an articular surface for the anterior end of the 

 palato-pterygoid. 



b. Palato-pterygoid. — Tliis is a flattened bone, somewhat in the form of a narrow 

 parallelogram, l)ut with the anterior end rounded. It is set in the skull obliquely to the 

 longitudinal axis and extends from the vomer to the quadrate, articulating with both and 

 with no other bones. Its alveolar portion is small and confined to a short curved ridge 



upon the outer border at the anterior end. The space 

 between the inner border of this bone and the outer border 

 of the parabasal is in the form of a very narrow triangle 

 and is filled in the recent state by a firm membrane. The 

 orientation ot this bone may be easily made by the teeth 

 which are upon the ventral side, at the anterior end, and 

 along the outer margin. 



The Visceral Skeleton. 



<v 



DORSAU 



VENTRAL 



Fig. 18. Two views of riglit pal- 

 alo-pterygoideum. X 4, (from .small 

 specimen). Contact .surfaces with 

 other bones are designated by an x. 



General morphology of the arches. — The seven visceral 



arches inherited from the Selachians, and sur\iving in most 



higher vertebrates, are represented here, although one of 



them (the (Ith) is vestigial, and appears in the adult as 



merely an intermuscular septum. Their disposition is as 



follows : — 



1. This arch, forming the functional jaws of the Selachians, possesses typically 



a dorsal and a ventral segment, of which the former is represented by the cartilaginous 



palato-quadrate arch, and the latter by Meckel's cartilage of the mandible. Both of these 



arches, in most higher vertebrates, become encased by dermal bones and lose their 



identity more or less completely in the adult, the palato-quadrate arch suffering more 



in this particular tlian tlie other. In Necturus the anterior and posterior ends of the 



palato-quadrate arch survive as the antorbital process and the quadrate cartilage 



(including its ossification) respectively, and the median portion is unrepresented. The 



