504 PISCES— FISHES. 



in connection with these arches do not yet form an integral 

 part of the skull. 



The toothed premaxilla forms the upper part of the gape, 

 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. 



There are four pairs of complete branchial arches, which 

 are divided into various parts. Of these the most interest- 

 ing 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 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 almost meets 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 

 innominate 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 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. 



