42 



PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, 



quadrate apparatus such as that exhibited by Cestracion. The huge 

 palate-quadrate cartilage (PI, Qu) of Cestracion is united with the 

 skull in the prseorbital region by a joint, and in the orbital region by 

 fibrous tissue, and answers to that part of the palato-quadrate cartilage 

 of Chimcera which lies between the nasal capsule and the mandible. 



Fig. 8. 



Hck 

 Cestracion philippi. Left lateral view of the skull. 

 a, occiput ; b, postorbital process ; c, c', antorbital process ; d, anterior end of 

 the chondrocraniuni ; ol, olfactory capsule ; Ot.p, otic process, or spiracular 

 cartilage; Sp, place of the spiracle; H.M, hyomandibular cartilage; Qu, 

 articulation of the palato-quadrate cartilage (PI, Qu) with the lower jaw 

 (Mok) ; p, part of the palato-quadrate arch which answers to the pedicle of 

 the suspensorium in Amphibia ; Hy, hyoid ; II, foramen for the optic, and V, 

 for the trigeminal nerves ; 1, 2, 3, 4, the upper and lower labial cartilages ; 5, 

 a small cartilaginous style attached by ligament to the mandibular cartilage. 



The small cartilaginous plate (Ot.jj), which is connected only by 

 ligament with the periotic cartilage above and with the quadrate 

 below, answers to the otic process of the Frog's suspensorium. This 

 cartilage lies in the front wall of the spiracle, which in Cestracion is 

 situated low at the sides of the head, nearly in a line with the bran- 

 chial clefts, or in the position which it occupies in foetal Selachians. 

 Moreover this so-called spiracular cartilage bears a rudimentary 

 gill and is so far comparable to any of the branchial arches*. 



In possessing this permanent mandibulo-hyoid cleft, or spiracle, 



which is the homologue of the tympanic cavity and Eustachian tube 



of the higher Vertebrata, and in the permanence of its rudimentary 



* Gegenbaur considers the spiracular cartilage to be a ray of the mandibular 



arch. 



