THE SKELETON IX FISHES. THE SKULL. 121 



beyond the normal series, thus in Hexanchus there are six, and 

 in Heptanchus seven. There are six also in Chlamydoselache 

 and Protopterus. 



4. The so-called external branchial arches which are carti- 

 laginous rods attached to all the visceral arches. They are 

 especially large in Cestracion. 



The skull in HOLOCEPHALI is entirely cartilaginous. The 

 palato-pterygo-quadrate bar is fixed to the cranium, and to 

 it the mandible articulates. There is a well-marked joint 

 between the skull and the spinal column. 



In living Cartilaginous GANOIDS the primitive cartilaginous 

 cranium is very massive, and is greatly prolonged anteriorly, 

 while posteriorly it merges into the spinal column. Although it 

 is mainly cartilaginous a number of ossifications take place in 

 the skull, and membrane bones are now found definitely de- 

 veloped, especially in connection with the roof of the cranium. 

 In Acipenser (fig. 18) the ossifications in the cartilage include 

 the pro-otic, which is pierced by the foramen for the fifth nerve, 

 the alisphenoid, orbitosphenoid, ectethmoid, palatine, pterygoid, 

 meso-pterygoid, hyomandibular (fig. 18, 11), cerato-hyal, all the 

 cerato-branchials, and the first two epi-branchials. Most of 

 these structures are, however, partly cartilaginous, though 

 they include an ossified area. The membrane bones too of 

 Acipenser are very well developed, they include a bone occu- 

 pying the position of the supra-occipital, and form a complete 

 dorsal cephalic shield. Resting on the ventral surface are a 

 vomer and a very large parasphenoid (fig. 18, 3). There is a 

 bony operculum attached to the hyomandibular, and membrane 

 bones representing respectively the maxilla and dentary are 

 attached to the jaws. The suspensorium is most markedly 

 hyostylic. The palato-pterygo-quadrate bar has a very curious 

 shape and is quite separate from the cranium. It is connected 

 to the hyomandibular by a thick symplectic ligament con- 

 taining a small bone homologous with the symplectic of 

 Teleosteans. 



