76 ANATOMY OF VERTEBRATES. 



to the end of the tympanic bone : the palatines are extremely small. 

 The premaxillary and maxillary bones seem to have coalesced ; 

 they expand as they extend backward to become attached to the 

 cartilage supporting the mandibular arch. The slightly ossified 

 pterygoids run parallel with them along the inner sides to the same 

 part. The articular and dentary pieces of the lower jaw have coa- 

 lesced, but there is a trace of a slender splenial piece on the inner 

 side of the mandible. All the bones of the mouth are edentulous, 

 but the membrane covering the extremities of the upper and 

 lower jaw is roughened by extremely minute denticles in the 

 recent fish. The ceratohyals are partially ossified : the rest of 

 the hyoidean arch is cartilaginous. A branchiostegal appendage 

 in the form of an irregular elongated flattened bone, resolved 

 posteriorly into osseous fibres, extends from each side of the 

 commencement of the hyoidean arch. A similar but larger oper- 

 cular appendage extends backward from the extremity of the 

 tympanic pedicle. 



27. Skull of Plagiostomi.- -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 different, apparently simpler type ; and 

 one which, through the progress of developement in the direct 

 vertebrate route, more nearly approximates to the cranial organi- 

 sation in the Batrachia. The Monk-fish ( Squatina, an interme- 

 diate form between the Sharks and Rays,) affords a good and typical 

 example of the essential characters of the plagiostomous skull. 

 The cranial end of the notochord and its capsule are converted 

 into firm granular cartilage ; extending forward so as to constitute 

 an oblong flattened plate forming the whole basis cranii. The 

 posterior margin of this 6 occipito-sphenoidal ' plate supports two 

 convex condyles, for articulation with the body and parapophyses 

 of the axis. The body of the atlas has coalesced with the basi- 

 occipital, as is indicated by its slender but separate neural arch. 

 The lateral margins of the basal cartilage have two notches, the 

 intervening prominence representing the primitive sphenoidal 

 arch, here filled up and sending off a rudimental pterygoid process 

 outwards. Just anterior to the median ridge there is a small 

 fossa, (in the young Squatina a foramen,) the last trace of the 

 pituitary canal : the basal cartilage then expands to form the 

 lower border of the groove which receives the palatine process or 

 point of suspension of the palato-maxillary arch, in front of which 

 it contracts to form the vomerine base of the cranium. The cra- 

 nial cavity is not moulded on the brain, but is of larger size ; it 



