242 



ZOOLOGY 



lab 



pr.orb 



sb.or 



Lam. 



art 



SECT. 



cartilage of the posterior 

 part are a pair of small 

 replacing bones which 

 appear to represent the 

 most anterior of the 

 spinal elements fused 

 with the skull. On the 

 upper surface are two 

 unpaired (A and B) and 

 four paired (C and sq.) 

 investing bones, the 

 homologies of all of 

 which are undetermined. 

 Premaxillse, maxillae, 

 and nasals are absent. 

 On the ventral surface 

 is a large investing bone 

 (Fig. 917, P.Sph.) repre- 

 senting the parasphenoid 



In 



FIG. 916. Ceratodus forsteri. Dorsal view of the ^ 

 skull. A, anterior median investing bone ; art. articular 



surface for second fin-ray ; , posterior median investing front IS a pair of Small 

 bone ; C, inner lateral investing bone ; lab. labial car- , , I -. 



tilages ; lam. process projecting over gills ; op. pper- upper labial Or nasal 

 culum ; pr. orb. pre-orbital process of chondrocranium ; ir f;i Q rrac A -nalaf/-. 



sb. or. sub-orbital bones ; sq. outer lateral investing Cdlllldges. J\. paidl 



bone. (After Huxley.) quadrate cartilage (Fig. 



915, pal.), firmly fixed to the side-wall of the cranium, gives attach- 

 ment to the mandible, so that 

 the skull is autostylic ; the quad- 

 rate element is distinct in the larva 

 and independently developed. 

 In front the palatoquadrate 

 contains a palatopterygoid ossi- 

 fication which forms the support 

 for the large composite tooth of 

 the upper jaw. The mandible 

 consists of Meckel's cartilage with 

 an angular bone behind, and a 

 large splenial, which bears the 

 tooth, in front. The dentary is 

 vestigial. The hyomandibular is 

 only represented by a small 

 vestige. Opercular (op.) and in- 

 teropercular (int.) bones support 

 the operculum. The hyoid (hy.) 

 and branchial arches (br.) are 

 cartilaginous. Of the latter, four 

 are completely developed, and a 

 fifth is represented by a vestige. 



FIG. 917. Ceratodus forsteri. Ventral 

 view of the skull, c, occipital rib ; d, 

 pa'atine teeth ; d' ', vomerine teeth ; na. 

 anterior and posterior nares ; P. palatine 

 region of palatopterygoid ; P. Sph. para- 

 sphenoid ; Pt. pterygoid ; Qu. quadrate 

 region ; Vo. vomer. (From Dean, after 

 Uiinther.) 



