THE SKULL OF FISHES. 73 



only its upper and middle part membranous, as in the human embryo 

 when ossification of the cranium commences. Two broad cartilages 

 {ib. 20, 21) may represent, upon the roof of the infundibular suctorial 

 mouth, the palatine and maxillary bones, and anterior to these there 

 is a labial cartilage {ib. 22) : there are likewise cartilaginous processes 

 ib. r, s) for the support of the large dentigerous tongue, and the 

 attachment of its muscles ; besides the cartilaginous basket, before de- 

 scribed, which supports the modified and perforated homologue of the 

 large respiratory pharynx in the Branchiostoma {fig- 46.). 



As regards the development of the skull, properly so called, the 

 ordinary course is pursued with very little deviation in the Der- 

 mopterous fishes ; but is arrested at more or less early embryonic 

 stages : yet at each of these, even the earliest, development proceeds 

 in a special direction, to stamp the species with its own distinctive 

 and peculiar character : in the Branchiostoma by the articulated 

 cartilaginous labial ai"ch and its numerous filaments ; and in the pro- 

 per Myxinoids and Lampreys by the formation of the complex system 

 of lateral and labial cartilages ; or by the modification of the palatine, 

 maxillary, and hyoid rudiments, in relation to the suctorial function 

 of the mouth. 



The more or less cartilaginous skull of the Plagiostomous fishes 

 might be histologically regarded as the transitional step from the 

 Cyclostomous to the Osseous fishes ; but, morphologically, it offers a 

 very different type, apparently a simpler one, if compared with the 

 Myxine or Lamprey, but one which in consequence of the progress 

 of development in the direct vertebrate route, more nearly approxi- 

 mates to the type of cranial organisation in the lower forms of Eep- 

 tilia. The Monk-fish {Sqimtina, — an intermediate foi-m between the 

 Sharks and Rays) affords a good and typical example of the essential 

 characters of the plagiostomous skull. The cranial end of the chorda 

 dorsalis and its capsule are converted into firm granular cartilage ; 

 and this cartilage extends from the prominent median basal ridge, 

 indicative of the primitive place of the chorda, on each side and for- 

 wards so as to constitute an oblong flattened plate forming the whole 

 basis cranii. The posterior margin of this ' occipito-sphenoidal ' 

 plate supports two convex condyles, as in most of the Rays, for arti- 

 culation with the body and jjarapophyses of the axis.* The lateral 

 margins of the basal cartilage have two notches, the intervening pro- 

 minence representing the primitive sphenoidal arch, here filled up 

 and sending off a rudimental pterygoid process outwards. Just an- 



* The body of the atlas has coalesced with the basi-occiphal, as is indicated by 

 its slender but separate neural arch. The condyloid foramen is just above the outer 

 end of the condyle. 



