xm PHYLUM CHORDATA 81 



mesethmoid, the optic nerves (Nv. 2) through or immediately 

 behind the orbito-sphenoids, the fifth nerves (Nv. 5) through or 

 immediately behind the alisphenoids, and the tenth nerves (Nv. 10) 

 through or immediately in front of the ex-occipitals. 



It will be seen that a clear distinction can be drawn between 

 the primary cranium or chondrocranium, formed by the fusion of the 

 parachordals, auditory capsules, and trabeculae, and consisting of an 

 undivided mass of cartilage more or less replaced by bones, and 

 the secondary cranium or osteocranium, modified by the super- 

 addition of investing bones. 



A similiar distinction may be drawn between the primary and 

 secondary jaws. The primary tipper jaiu, or palato-quadrate, be- 

 comes ossified by three chief replacing bones on each side, the 

 palatine (A, PAL.) in front, then \\iorpterygoid (PTG.), and the quad- 

 rate (QU.) behind, the latter furnishing the articulation for the 

 lower jaw or mindible. In the higher classes the primary upper 

 jaw does not appear as a distinct cartilaginous structure, and the 

 palatine and pterygoid are developed as investing bones. The 

 secondary upper jaw is constituted by two pairs of investing bones, 

 the pre-maxilla (P. MX) and the maxilla (MX), which in bony skulls 

 furnish the actual anterior boundary of the mouth, the primary jaw 

 becoming altogether shut out of the gape. The proximal end of 

 the primary lower jaw ossifies to form a replacing bone, the articular 

 (ART.), by which the mandible is hinged : the rest of it remains as 

 a slender, unossified Meckcl's cartilage (Mck. C), which may dis- 

 appear entirely in the adult. The secondary lower jaw is formed by 

 a variable number of investing bones, the most important of 

 which is the dentary (DNT). In Mammalia the dentary forms the 

 entire mandible, and articulates, not with the quadrate, but with 

 a large investing bone formed external to the latter, and known 

 as the squamosal (SQ). 



In the hyoid arch a replacing bone, the hyj-mandibular (HY. M), 

 appears in the cartilage of the same name, and ossifications are 

 also formed in the various segments of the hyoid cornua (EP. HY, 

 c. HY, H. HY, B. HY) and of the branchial arches (PH. BR, EP. BR, 

 c. BR, H. BR, B. BR). In the air-breathing forms both hyoid and 

 branchial arches undergo more or less complete atrophy, the whole 

 gill-bearing apparatus becoming reduced mainly to a small hyM 

 lone serving for the support of the tongue. 



The skeleton of the median fins is formed of a single row of 

 cartilaginous rays orpterygioplwres (Fig. 760, C and D,/.r), lying in 

 the median plane, and more numerous than the vertebrae. They 

 may ossify, and may be supplemented by dermal fin-rays, 

 of varying composition, developed in the derm along the free edge 

 of the fin. The latter are clearly exoskeletal structures. 



Both pectoral and pelvic fins are supported by pterygio- 

 phores or radialia (Fig. 766, Had.) the basal or proximal 



