434 FISHES. 



while the maxilla which articulates dorsally with the vomer, 

 and nearly reaches the quadrate posteriorly, does not enter 

 into the gape. Both are membrane bones. 



In the opercular fold are four membrane bones. 



The branchial arches are divided into various parts, of 

 which the most interesting are the two superior pharyngeal 

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

 and their counterpart, the inferior pharyngeal bone, which 

 lies on the floor of the pharynx, and is likewise toothed. 



The Limbs and Girdles. — The fin-rays of the pectoral 

 fin are attached to four small brachial ossicles ; these articu- 

 late with a dorsal scapula and a more ventral coracoid ; 

 both of these are attached to the inner face of a large 

 clavicle, which curves to meet its fellow of the other side 

 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 fin-rays of each pelvic fin are attached to a thin in- 

 nominate bone, which may be a basal element of the fin, or 

 the rudiment of a pelvic girdle. 



Nervous System. — The relatively small cerebral hemi- 

 spheres, the thalamencephalon with its inferior lobes and 

 infundibulum, the large optic lobes, the tongue-shaped cere- 

 bellum which conceals most of the medulla oblongata, have 

 their usual 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. 

 But the relations of these and the rest may be inferred from 

 our description of the skate. 



In the large eyes, the different parts will be readily identi- 

 fied; the small nasal sacs with plaited walls have double 

 anterior apertures ; the vestibule of the ear contains a large 

 otolith, and another very small one in a posterior chamber. 

 The black lateral line, covered over by modified scales, lodges 

 sensory tubes, and is innervated by a branch of the vagus. 



