8 



arranged and how far forward do they extend? Are all of equal 

 size? 



The pterygoquadrate bears near the middle of its dorsal 

 surface a strong palatal process which extends dorsally to articu- 

 late with the basilar process of the cranium. Near the angle the 

 upper jaw is expanded into a large quadrate process, excavated 

 externally for the attachment of muscles and produced downward 

 for the articulation of Meckel's cartilage. 



MeckePs cartilage presents no special features except the 

 articular surface of the angle for articulating with the pterygo- 

 quadrate. 



(There is a spiracular cartilage in the anterior wall of the 

 spiracle. It is lost in preparing a skull in the ordinary way.) 



The hyoid arch also consists of dorsal and ventral portions, 

 the dorsal being the hyomandibular and the ventral the hyoid 

 proper. The two are hinged together and are connected by 

 ligament with the Meckelian. The hyomandibular also articulates 

 with the cranium, and hence the jaws are indirectly connected 

 with the rest of the skull, the hyomandibular forming the sus- 

 pensorioim. 



The hyoid proper lies behind and between the two halves 

 of the lower jaw. It consists of a pair of ceratohyals united 

 medially by an unpaired basihyal. On the posterior margin of 

 both ceratohyal and hyomandibular are a number of carti- 

 laginous branchial rays which support the gill. 



Study the second branchial arch. Make out from above 

 downward in each half four cartilages: (1) a pharyngobranchial 

 lying in the roof of the pharynx and directed backwards; (2) an 

 epibranchial forming the rest of the upper half of the arch; (3) a 

 ceratobrancnial; and (4) a small backwardly curved hypobranchial 

 in the floor of the pharynx. Which of these elements bear 

 branchial rays? Why are they not present on all? Also note 

 on the inner surface of the arch shorter cartilages supporting 

 the 'gill-strainers'. Lying external to the ends of the branchial 

 jays but approaching the arch above and below are a dorsal and 

 a ventral extrabranchial cartilage (better seen when dissecting 

 the gills). 



Compare the other branchial arches with this one. Do all 

 have the same parts? Note how the first ceratobranchial artic- 

 ,ulates with the hyoid arch, and that the first hypobranchial 



