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.x~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 
Jusion 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 premaxille, 
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- 
