Chap, ix.] MOUTH OF CYCLOSTOMATA. 335 



sides, and has behind it a ridge of bone on the orbito- 

 sphenoid, and frontal bones by which it is separated 

 from the cerebral fossa, which, in all Mammals, 

 occupies a larger part of the cranial cavity ; this fossa 

 is more or less feebly divided into two by a ridge of 

 bone which corresponds to the sylvian fissure of the 

 brain ; in the more anterior division there lies the 

 frontal lobe (see page 426) ; behind this comes the cere- 

 bellar fossa, marked off anteriorly by the tentoriuni. 

 The flocculus of the cerebellum lies in a special depres- 

 sion on the inner face of the periotic, and the hypo- 

 physis cerebri on a pit (sella turcica) on the 

 superior face of the basisphenoid, which, as we have 

 already learnt, forms a portion of the floor of the brain 

 cavity. 



Such, then, being the general disposition of the 

 parts of the skull in a well-developed Vertebrate, we 

 have now to investigate the arrangements which obtain 

 in various forms in relation to their habits of life and 

 their zoological affinities. 



In the Round-mouths, where no branchial rods 

 are modified to form jaws, the sides of the buccal orifice 

 are supported by cartilaginous pieces, the so-called 

 labial cartilages ; the mouth is surrounded by a circu- 

 lar lip, in the posterior region of which is placed the 

 annular cartilage ; into the cavity of the mouth 

 there project a number of horny denticles ; on the 

 floor is the lingual cartilage, and in front of this is the 

 median ventral cartilage, which, possibly, represents 

 the basal median portion of the mandibular arch, which 

 in other Vertebrates is only to be detected in early 

 stages. It is very instructive to observe that there is 

 a close resemblance between the mouth parts of a 

 lamprey and those of a tadpole during the period when 

 the latter has a suctorial mouth. 



The labial cartilages of the Cyclostomata appear to 

 be better retained by Elasmobranchs than by other 



