THE SKULL 81 



The dorsal segments of these arches become fused together to 

 form the "superior pharyngeal bone," which, like the inferior 

 pharyngeal, usually bears teeth. 



A curious asymmetry is seen in the head of adult Pleuronectidce. When 

 hatched, these Fishes are quite symmetrical, but later on the eye of one side 

 becomes rotated, so that eventually both eyes are situated on the same side ; 

 in consequence of this, the skull also becomes asymmetrical. 



The tactile barbules present on the head of many Fishes (e.g., Siluroids) 

 are supported by skeletal parts. 



u. Dipnoi. 



The skull of the Dipnoi is in a sense intermediate between that 

 of the Holocephali, Ganoidei, and Teleostei, on the one hand, and 



FIG. 62. CRANIAL SKELETON, PECTORAL ARCH, AND ANTERIOR EXTREMITY OF 



Protopterus. 



W, W l , the vertebrae which are fused with the skull, with their neural spines (Psp, 

 P$p l ) ; Occ, exoccipital, with the hypoglossal foramina ; Ob, auditorycapsule ; 

 Tr, trabecular region, with the foramina for the trigeminal and facial nerves ; 

 FP, f roil to-parietal ; Ht, membranous fontanelle, perforated by the optic 

 foramen (//) ; SK, supra-orbital ; SE, supra-ethmoid ; NX, cartilaginous 

 nasal capsule ; AF, antorbital process (the labial cartilage, which has a similar 

 position and direction, is not indicated) ; PQ, palatopterygoid, which converges 

 towards its fellow of the other side at PQ 1 ; Sq, squamosal, covering the 

 quadrate ; A, A 1 , articular, joined to the hyoid (Hy) by a fibrous band () ; 

 D, dentary ; ft, Meckel's cartilage, which is freely exposed, and grows 

 out into prominences ; SL, a, b, teeth ; Op, Op 1 , rudimentary opercular 

 bones ; 7 to V, the five branchial arches ; KR, cranial rib ; LK, MK, 

 lateral and median bony lamellae which ensheathe the cartilage of the 

 pectoral arch (Kn, Kn l ) ; co, fibrous band which binds the upper end of the 

 pectoral arch with the skull ; x, articular head of the pectoral arch, with 

 which the basal segment (b) of the free extremity articulates ; *,*, rudimen- 

 tary lateral rays of the extremity (biserial type) ; 1, 2, 3, the three next seg- 

 ments of the free extremity ; K, external gills. 



G 



