PISCES 



11 



the lower jaw are articulated with the palatoquadrate. Tavo small labial 

 cartilages, sometimes termed maxilla and premaxilla, occur on each side in 

 front of and below the palatoquadrate. The latter element is connected 

 with the skull in the ethmoidal region by ligament, and in the hinder portion 

 by the second visceral arch, of which the upper portion (hyomandibular) is in 

 contact with the auditory region of the skull, while the lower portion forms 

 the hyoid arch. The palatoquadrate and mandible are provided with teeth. 

 Behind the hyoid arch there follow five (rarely seven) other visceral arches, 

 which serve for the support of the respiratory organs, are composed of severtal 

 pieces, and united ventrally by median intercalary pieces (copulae). Both the 

 gill arches and the hyoid bear cartilaginous gill rays. In the Holocephali the 

 palatoquadrate and hyomandibular fuse together and with the cranial capsule. 

 The mandible thus becomes autostylic, i.e. articulates directly Avith the 

 cranium. 



In the cartilaginous ganoids (Acipenser, Spatularia), bony elements begin 

 to take part in the formation of the head. The cranial capsule, indeed, 

 remains for the most part cartilaginous ; but externally there are a number 

 of bony plates, which cover the cranium on the top and sides, and there 

 is also developed below the base of the skull a long, delicate, dermal bone 

 (parasphenoid), which extends forwards to the ethmoidal region and backwards 

 even beyond the occiput. The palatoquadrate, hyomandibular, and mandible 

 are also covered with bone ; and the fold of skin covering the gill clefts is 

 likewise ossified as a single gill cover (operculum). The palatoquadrate 

 and hyomandibular form the freely movable (hyostylic) supports of the 

 mandible. 



The Dipnoi resemble the cartilaginous ganoids, except that in them the 

 palatoquadrate and hyomandibular are fused with the cranium (autostylic). 



In the scaly ganoids and bony fishes a more or less complete ossification of 

 the cranial capsule and visceral 

 skeleton occurs. The ossifica- 

 tion begins first, as in the carti- 

 laginous ganoids, with dermal 

 bones, and then the substitution 

 of bony substance for cartilage 

 follows. The distinction between 

 dermal bones and ossified carti- 

 lage (cartilage bones) can, how- 

 ever, only be made by determin- 

 ing their mode of development. 



In the cranium proper (Fig. 

 23) three regions are distin- 

 guishable — the occipital portion, 

 the auditory, and the orbito- 

 ethmoidal regions. The hinder- 

 most or occipital region consists of four cartilage bones : — (1) The lower 

 occipital bone (occipitale basilare, basioccipital), which usually contains a 

 persistent remnant of the notochord, and is in contact Avith the vertebral 

 column with its deeply hollowed hinder end ; (2, 3) a right and left lateral 

 occipital (occipitalia lateralia, exoccipitals), which bound the greater part of 

 the foramen magnum ; and (4) an upper occipital (occipitale superius, supra- 



Cranium of the Carp {Cyprinus carpio, Linn.). AIS, Ali- 

 sphenoid ; BO, Basioccipital ; BSph, Basisphenoid ; Ep, Bpiotic ; 

 Eth, Etlimoid ; Exo, Exoccipital ; Fr, Frontal ; io, Interorbital ; 

 OpU, Opisthotic ; OSp)li, Orbitosphenoid ; Prf, Prefrontal ; PrO, 

 Prootic ; Ft/, Postfrontal ; SO, Supraoooipital ; Sq, Squamosal ; 

 Vo, Vomer (after R. Owen). 



