6.8 COMPARATIVE ANATOMY. 



the underlying hyaline primordial skull persists entirely (Cera- 

 todus), or to a large extent. Gill-covers and branchiostegal rays 



S!E 



FIG. 54. CRANIAL SKELETON, PECTORAL ARCH, AND ANTERIOR EXTREMITY OF 



Pi'otopterus. ~y^ v v Y\ t> 



ir, W l , the vertebrae which are fused with the skull, with their spinous processes 

 (Psp, Psp 1 ) ; Occ, supraoccipital, with the liypoglossal foramina ; Ob, auditory 

 capsule ; Tr, trabecula, with the foramina for the trigemmal- and facial nerves ; 

 FP, fronto-parietal ; Ht, membranous fontanelle, perforated by the optic foramen 

 (II) ; SK, supra-orbital ; SJE, supra- ethmoid ; NX, cartilaginous nasal capsule ; 

 AF, antorbital process (the labial cartilage, which has a similar position and direc- 

 tion, is not indicated) ; PQ, palatoquadrate, which converges towards its fellow 

 of the other side at PQ 1 j 8q, squamosal, covering the quadrate ; A, A 1 , articular 

 joined to the hyoid ( ffy) by a fibrous band () ; L>, external dentary ; ft, Meckel's 

 cartilage, which is freely exposed, and grows out into prominences ; SL, ena- 

 melled ridge ; a, b. teeth ; Op, Op r , rudimentary opercular bones ; / to VI, the 

 six branchial arches ; KR, cranial ribs ; LK, MK, lateral and median bony lamella?, 

 which ensheathe the cartilage of the pectoral arch (Kn, Kn 1 } ; 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 ; *,*, rudimentary lateral rays of the extremity (biserial type) ; 1, 2, 3, 

 the three next segments of the free extremity. 



are present, though greatly reduced, and even the five or six car- 

 tilaginous gill-arches are in a very rudimentary condition. The 

 sharp, blade-like teeth, covered with enamel, deserve notice. 



Teleostei. In this group, the skull presents a large amount 

 of variation ; its ground-plan, however, may always be derived from 

 that of the bony Ganoids, and more particularly from Amia 

 calva. On the other hand, no relations with the Amphibia are 

 observable, and we must consider the whole group of the bony 

 Fishes as a side branch of the piscine phylum. 



