GENERAL STRUCTURE. 



917 



842) shows, however, the upward bend of the notochord, although 

 this is very much less marked in the adult than in the young. This 

 masked heterocercal type is sometimes described as homocercal. 

 The coalesced hsemal spines found in this type of tail are known as 

 hypural bones (fig. 842, h), 

 while the ossified extremity 

 of the notochord is termed 

 the urostyle. 



The skulls of Fishes pre- 

 sent variations in regard to 

 their degree of ossification, 

 analogous to those obtaining 

 in the vertebral column. The 

 general structure of the pri- 

 mitive cartilaginous skull has 

 been already indicated in 

 Chapter xlv. (fig. 822), but 

 we must here glance briefly at certain bones developed in the more 

 specialised forms which are peculiar to the class. Thus, taking as 

 an example the skull of a Teleostean Fish (fig. 843), where the 

 primitive cranium is 



_P Of 

 CcuiSft.pfft p jjyj\ 



Fig. 842. — Tail of Flounder. (After Agassiz.) v, 

 Vertebral column ; n, Turned-up end of the noto- 

 chord ; h, Hypural bones. 



M>m 





m: 



concealed by the de- 

 velopment of investing 

 bones, we find two 

 peculiar ossifications in 

 the auditory region 

 known as the pterotic 

 {ibid., Pie), which is 

 considered to repre- 

 sent the squamosal and 

 opisthotic of higher 

 Vertebrates, and the 

 sphenotic {ibid., Sph). 

 A large parasphenoid 

 is always present infe- 

 riorly (fig. 844). The 

 intervention of the hy- 

 omandibular and sym- 

 plectic (figs. 823 and 

 843) between the quad- 

 rate and the squamosal 

 region has been already 



mentioned in Chapter xlv. as peculiar to Fishes (although it is by no 

 means universal in the class) ; and we must also mention that, in 

 addition to the normal palatine and pterygoid bones developed 



Fig. 843. — Left lateral view of the skull of a Trout (Sabno). 

 Ep, Epiotic ; Pte, Pterotic ; Sph, Sphenotic ; Os, Supra- 

 occipital ; P, Parietal ; F, Frontal ; Sp.eth, Ethmoid ; Can, 

 Aperture of olfactory nerve ; Nl, Nasal ; Pmx, Premaxilla ; 

 M, M 1 , Maxilla ; Ig, Jugal ; Ms, Mesopterygoid ; Mtp, 

 Metapterygoid ; o, o, Suborbitals ; Hm, Hyomandibular ; 

 s, Symplectic; Qu, Quadrate ; Pr, lop, Sop, Pre-, inter-, and 

 suboperculum ; Op, Operculum ; BsS, Branchiostegal rays ; 

 Ar, Articular; De, Dentary; A, eye. (After Wiedersheim.) 



