ANATOMY OF HETERODONTUS: ENDOSKELETON 455 
becular cartilage (ba.tr.) below would give to Crossorhinus a 
type of rostrum much like that of Heterodontus. 
b. The visceral skeleton is composed of a series of right and left 
cartilaginous arches which more or less completely surround 
the buccal cavity and the pharynx. These, in Heterodontus, if 
viewed, say, from the left side, are like those of other pen- 
tanchid sharks, seven in number; they may be divided into two 
groups. The first group comprises the first and second arches, 
Text-fig. A Nasal region of Heterodontus francisci and Crossorhinus barbatus; 
ba.tr., basi-trabecular cartilage; f.op.pr.’, ophthalmic foramen for profundus 
nerve; f.op.VII', ophthalmic foramen for facial nerve; Fo., anterior fontanelle; 
ol.wg., olfactory wing; rst., rostral cartilage. 
the first of which, the mandibular, is composed of the upper and 
the lower jaw; the second, the hyoid arch, is similarly made up 
of two segments. The second group consists of five branchial 
arches which support the pharynx. In structure the branchial 
arches are essentially similar to the first two arches excepting 
that in these there are typically four segments to anarch. These 
differ among themselves, however, in minor details. 
The mandibular, or first arch (fig. 7), has become the most 
highly specialized of all the visceral arches. Its upper segment, 
representing the palatal and quadrate regions, is called the 
