ANATOMY OF VERTEBRATES. 



73 



circumscribe elliptical spaces outside the presphenokl plate : these 

 appear to represent the pterygoid arches, fig. 61, z, but, as in the 

 embryo of higher fishes, are not separated from the base of the skull 

 by distinct joints. The basal cartilages, after forming 

 the ear-capsules, ib. g, extend upon the sides of the cra- 

 nium, ib. h, arch over its back part, and leave only its 

 upper and middle part membranous, as in the human 

 embryo when ossification of the cranium commences. 

 The cranium is continued below the olfactory capsule, 

 ib. k, into the ' rostral plate,' /. Behind the pterygoid 

 arch, z, the process representing the stylo-hyal, ib. i' 9 

 i" ', passes down, and expands to give attachment to 

 the muscles of the tongue ; the f basihyal ' supports, 

 by its forward f glossohyal ' extension, the large den- 

 tigerous tongue, and by its backward ' urohyal ' growth, s, adds to 

 the surface of insertion of the muscles. The cartilage descending 



o & 



from the side of the fore part of the cranium to join the pterygoid 

 arch, i, may represent a ' tympanic ' pedicle : it mainly supports, 

 as in the Sturgeon, fig. 62, ?8, and Shark, the membrane, fig. 61, x, 

 and cartilages, forming the roof and margin of the mouth ; in which 



Gl 



Base of skull, 

 Lamprey. 



XXI. 



Skull of Sea Lamprey (Tctromyzon marinus). xxi. 



n may be compared to the ' palatine,' and o to the maxillary, while 

 p seems to be a special labial cartilage in this suctorial fish : q and r 

 are processes for the muscles working this peculiar apparatus ; 

 and in addition to these is the cartilaginous basket before de- 

 scribed, fig. 24, 45, which supports the modified and perforated 

 homologue of the large respiratory pharynx, fig. 23, a, in the 

 Branchiostome. 



Thus, in the Dermopterous fishes, the developement of the skull 

 is arrested at more or less early embryonic stages ; whence it 



