XIII 



PHYLUM CHORDATA 



249 



absent. On the ven- 

 tral surface is a large 

 investing bone (Fig. 

 906, P. Sph.) represent- 

 ing the parasphenoid 

 of the Teleostomi. In 

 front is a, pair of small 

 upper labial or nasal 

 cartilages. A palato- 

 quadrate cartilage (Fig. 

 904, >/.), 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 in- 

 dependently developed. 

 In front the palato- 

 . quadrate contains a 

 palatopterygoid ossifi- 

 cation which forms the 

 support for the large 

 composite tooth of the upper jaw. 



art 



FIG. 905. Ceratodus forsteri. Dorsal view of the 

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

 surface for second fin-ray ; B, posterior median investing 

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

 tilages ; lam. process projecting over gills ; op. oper- 

 culum ; pr. orb. preorbital process of chondrocranium ; 

 sb. orb. suborbital bones ; *q. outer lateral investing 

 bone. (After Huxley.) 



FIG. 906. -Ceratodus forsteri. Ventral 

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

 palatine teeth ; d', vomerine teeth ; na. 

 anterior and posterior nares ; P. palatine 

 region of palatopterygoid; P. *ph. para- 

 sphenoid ; Pt. pterygoid ; Qu. quadrate 

 region ; Vo. vomer. (From Dean, after 

 Giinther.) 



The hyomandibular is only 

 represented by a small vestige. 

 Opercular (op.} and interoper- 

 cular (int.) bones support the 

 operculuni. The mandible con- 

 sists of Meckel's cartilage with 

 an angular bone behind, and a 

 large splenial, which bears the 

 tooth, in front. The dentary is 

 vestigial. The hyoid (hy.) and 

 branchial arches (5r.) are cartil- 

 aginous. Of the latter, four are 

 completely developed, and a fifth 

 is represented by a vestige. There 

 are no branchial rays, but the 

 branchial arches bear a series of 

 gill-rakers with cartilaginous 

 supports. 



The pectoral arch (Fig. 904, 

 pet.) is a stout cartilage with two 

 pairs of investing bones, the 

 clavicles on the coracoid, and the 

 cleithra on the scapular regions. 



