THE SKULL OF OSSEOUS FISHES. 95 



nerves. The ali-sphenoid articulates in tlie Cod with tlie petrosal 

 posteriorly, with the oi'bito-sphenoid anterioi'ly, and with the mastoid 

 and post-frontal above. Where the ali-sphenoids have a greater 

 relative size, as in the Perch, and where the less constant petrosal 

 decreases or disappears, their connections are more extensive ; 

 they then reach the ex-occipitals, and sometimes even join a 

 small part of the basi-occipital. In the incompletely ossified skulls 

 of some fishes, e. g. the Pike and the Salmon tribe, the basal and 

 lateral cranial bones are lined by cartilage, which forms the medium 

 of union between them, especially the lateral ones : in better ossified 

 fishes, e. g. the Cod, the union of the ali-sphenoids is by suture, partly 

 dentated, partly squamous. In the Cod the second and third di- 

 visions of the trigeminal nerve pass out of the cranium by the an- 

 terior notch ; in some other fishes they escape by foramina in the 

 ali-sphenoid : a part of the vestibule and the anterior semicircular 

 canal of the acoustic labyrinth usually encroach upon its inner con- 

 cavity, whence some have deemed it to be the petrous bone.* 



The parietals (spine of mesencephalic arch, Jtgs. 30. 32. 7), which 

 complete above the osseous cincture of the most expanded segment of 

 the brain in fishes, are most commonly two in number: in the Cyprinoid 

 {fig. 35, 7) and Salamandroid fishes they meet and unite by a sagittal 

 suture ; in the Salmonoids they soon coalesce; and in some Siluroids not 

 only with each other but also with the supra-occipital : in the Pike, the 

 Perch, the Cod, and most osseous fishes, the parietals are separated 

 from one another by the anterior prolongation of the supra-occipital. 

 They are always flat, and present much smaller proportions than in the 

 higher classes of Vertebrata. They are commonly articulated to the 

 mastoids outwardly and below, to the supra-occipital above, to the 

 frontal before, and to the par-occipital behind ; sometimes, but rarely 

 to the ali-sphenoids, and in a few fishes, as the Pike and Gurnard, 

 where the parietals are more than usually developed, they appear upon 

 the hinder as well as the upper surface of the skull. In some fishes 

 they are perforated by the nervus lateralis which supplies the ver- 

 tical fins. The left parietal is broader than the right in the Holibut 

 and some other flat fishes {Pleuroncctidcr). The parietals are ossified 

 in and from the perichondrium and continuous membrane closing the 

 great fontanelle of the primitive cartilaginous cranium. 



The mastoids (parapophyses of the parietal vertebra, ib. 8) bear 



* As, e. g. Meckel, Wagner and Ilallinan ( Vcrf;leicliende Osteologie dcs 

 Schlafenbcincs, p. 55.). Kostliii, who approves ot" tliis view, gives, however, 

 the name of posterior ali-sphenoid (liintern schliifen-fiugel, xxxv, p. 31.'>.) to the 

 petrosal. 



