4 8 



MORPHOLOGICAL TYPES 



spiracle. The upper and lower portions of this arch's skeleton 

 move on one another and form the upper and lower jaws. 

 The skeleton of the upper jaw is the ptery go- quadrate, that of 

 the lower is Meckel's cartilage. The possession of jaws is the 

 criterion of the group Gnathostomata, to which Scyllium and 

 all higher forms belong. The arches between the gill-slits 

 also have cartilaginous rods. That of the second or hyoid 



i ,fj Sv qc. 



Fig. 25. — Scyllium : view of skull and visceral arches. 



a, foramen for efferent pseudobranchial artery ; ac, auditory capsule ; 

 c, centrum of vertebra ; ch, ceratohyal ; cb 2, ceratobranchial of second 

 arch ; d, foramen for dorsal spinal nerve-root ; eb 4, epibranchial of fourth 

 arch ; hb 2, hypobranchial of second arch ; hm, hyomandibula ; id, inter- 

 dorsal cartilage ; Mc, Meckel's cartilage ; o, orbit ; oc, olfactory capsule ; 

 pb 1 , pharyngobranchial of first arch ; pg, pterygoquadrate cartilage ; 

 v, foramen for pituitary vein ; vr, foramen for ventral spinal nerve-root ; 

 II, optic nerve foramen ; III, oculomotor nerve foramen ; IV, trochlear 

 nerve foramen ; V and VII, trigeminal and facial foramen ; V and VII o, 

 foramina for ophthalmic branches of trigeminal and facial. 



visceral arch (separating the spiracle from the 1st gill-slit) is 

 composed of a dorsal portion, the hyomandibula ; and a 

 ventral portion, the ceratohyal, and basihyal. The following 

 visceral arches are made up of four pieces on each side, which 

 are from above downwards, the pharyngobranchial, epi-, 

 cerato-, and hypobranchial. There is also a median basi- 

 branchial. The pterygo-quadrate and the hyomandibula 

 represent the " epi " elements of their respective arches, 



