NERVOUS SYSTEM. 555 



In the opercular fold are four membrane bones. 



There are four pairs of complete branchial arches, which are divided 

 into various parts. Of these the most interesting are the two superior 

 pharyngeal bones, which lie in the roof of the pharynx and bear teeth, 

 and are formed by the coalescence of the dorsal elements of the arches. 

 Their teeth bite against those of the inferior pharyngeal bones, which 

 lie on the floor of the pharynx, and represent the fifth branchial arches. 



The limbs and girdles. The dermal rays of the pectoral fin are 

 attached to four small brachial ossicles ; these articulate with a dorsal 

 scapula and a more ventral coracoid ; both of these are attached to the 

 inner face of a large clavicle or cleithrum, which almost meets its fellow 

 in the mid-ventral line of the throat. From the clavicle a slender 

 post-clavicle extends backwards and downwards ; while a stout supra- 

 clavicle extends from the dorsal end of the clavicle upwards to 

 articulate with a forked post-temporal, which articulates with the back 

 of the skull. It must not be assumed that the elements of this girdle 

 are directly comparable to those of a higher Vertebrate, although the 

 nomenclature is the same. 



The pelvic girdle seems to be absent, as in almost all Teleostomes, 

 but its place is taken by a thin plate of bone, apparently due to a 

 fusion of some basal elements of the pelvic fins. 



Nervous System. The relatively undifferentiated fore- 

 brain with defective cortical region, the thalamencephalon 

 with its inferior lobes and infundibulum, the large optic 

 lobes, the tongue-shaped cerebellum which conceals most 

 of the medulla oblongata, have their usual general relations. 

 Each of the olfactory nerves is at first double ; their bulb- 

 like terminations lie far from the brain behind the nasal 

 sacs. The large optic nerves cross one another without 

 fusion at a slight distance from their origin ; otherwise the 

 nerves generally resemble those of the skate. 



The eyes are large but lidless ; the small nasal sacs with 

 plaited walls have double anterior apertures ; the vestibule 

 of the ear contains a large solid otolith, and another very 

 small one in a posterior chamber. The dark lateral line, 

 covered over by modified scales, lodges sensory cells, and 

 is innervated by a branch of the vagus. 



Alimentary system. Teeth are borne by the premaxillae, 

 the vomer, and the superior pharyngeal bones above, by 

 the dentaries and the inferior pharyngeal bones beneath. 

 There are no salivary glands, no spiracles, nor posterior 

 nares. A small non-muscular tongue is supported by a 

 ventral part of the hyoid arch. Five gill-clefts open from 

 the pharynx ; their inner margins are fringed by horny gill- 



