82 MORPHOLOGICAL TYPES 



in which the upper jaw is fastened on to the skull. The 

 quadrate is directly attached to the neurocranium by carti- 

 laginous processes : a basal process and an otic process. 

 There is also an ascending process. This method of suspension 

 of the jaws is called autostylic ; the hyomandibula plays no 

 part in it. The relations of the basal, otic and ascending 

 processes to the neighbouring nerves, veins, and arteries are 

 important, and most of them will be found to be identical 

 in all the remaining groups of vertebrates. The ascending 

 process lies between and separates the profundus from the 

 maxillary branch of the trigeminal {i.e. it is situated between 

 Y 1 and V 2 ) ; the palatine nerve runs down behind and forwards 

 beneath the basal process ; while the facial nerve (hyomandi- 

 bular branch) and jugular vein pass on the inner and under 

 side of the otic process. 



The premaxilla and maxilla have disappeared, and conse- 

 quently there are no teeth round the edge of the jaws. In 

 the lower jaw the dentary is very much reduced. Teeth are 

 carried on the prevomer, pterygo-palatine, and splenial (mem- 

 brane-bone). An angular is present in the lower jaw. 



In the hyoid arch there are a very small hyomandibula, 

 and well- developed ceratohyal, hypohyal, and basihyal. The 

 skeleton of the branchial arches also is not very well developed, 

 and the arches do not carry any branchial rays. 



The operculum is supported by opercular and subopercular 

 bones. 



Vertebral Column. — The vertebral column, which is con- 

 tinuous in front with the hind end of the skull, is made up of 

 paired basidorsal and basiventral cartilaginous elements, which 

 do not interrupt the notochord. The basidorsals rise up into 

 bony neural arches and neural spines which are attached to the 

 jointed radials supporting the median fin. The basiventrals 

 in the hinder region form haemal arches carrying haemal spines 

 and radials supporting the ventral median fin. Farther 

 forwards the basiventrals are produced into ribs. These do 

 not extend into the horizontal septum between the dorsal and 

 ventral portions of the myotomes, like the true or " dorsal " 

 ribs of Scyllium. Instead, they bend down and lie just 



