THE SKULL OF OSSEOUS FISHES. 91 



tribe (fig. 35. 2), where they relate also to the connection of the air- 

 bladder with the organ of hearing, by means of the ossicles «, h, c, d, 

 and e. The ex-occi[)itals are immovably articulated in the Cod, 

 below with the basi-occipital, behind with the neurapophyses of the 

 atlas, above with the supra-occipital and the par-occipitals, and in 

 front with the petrous bones, or acoustic capsules, intercalated between 

 them and the alisphenoids. In a few fishes (e. g. Fistularia) the ex- 

 occipitals send backwards articular processes modified to allow a 

 slight movement upon the corresponding anterior articular processes 

 of the neurapophyses of the atlas. Like these elements of the ordinary 

 vertebrae of some fishes (e. g. Lepidosiren, Thynniis, XypJiias), the 

 bases of the ex-occipitals expand, approximate, and in most osseous 

 fishes, meet upon the upper surface of the basi-occipital, and imme- 

 diately support the medulla oblongata ; but sometimes a space is left 

 between them, which is filled up by the basi-occipital *, and in Lepid- 

 osteus, as I have just obsei'ved, the basi-occipital protects the whole 

 epencephalon. 



The supra-occipital (spine of the occipital vertebra, ^^. 30 & 31. 3), 

 of an elongated rhomboidal form in the Cod, triangular in the Carp, is 

 articulated by an inferior cellulo-sutural surface, with the summits of 

 the ex-occipitals and the mesial angles of the par-occipitals, com- 

 pleting the circle or forming the key-stone of the neural arch : it 

 usually sends upwards and backwards a strong compressed spine from 

 the whole extent of the middle line, and a transverse ' supra-occi- 

 pital ' ridge outwards from each side of the base of the spine, to the 

 external angles of the bone. In most fishes this bone advances for- 

 wards and joins the frontal, pushing aside as it were the parietals : 

 in Balistes, the produced part of the supra-occipital is even wedged 

 into the hinder half of the frontal suture. In the Carp, on the conti'ary, 

 the anterior angle of the supra-occipital is truncated, forming the base 

 of the triangle, and is articulated by a lambdoidal suture to the parietal 

 bones, (Jig. 35. 7), which here meet at the mid-line of the skull, and 

 the upper part of the occipital spine is low and flattened. The 

 supra-occipital is also separated from the frontal by the parietals, in 

 the Salmonoid, Clupeoid, Murainoid, and Salamandroid fishes (Lepid- 

 osteus, Polgpterus), and is itself divided, in Lepidosteus, by a median 

 suture ; these modifications tell strongly against extending the honio- 



* INI. Agassiz, who has noticed a similar interspace between the summits of the 

 ex-occi))itals, as well as between the par-occipitals and siir-occipital above, observes, 

 " On dirait alors qu'iine large fente mediane cntame tout I'occiput." ( Poissons 

 Fossiles, i. p. 118.) But this could only be affirmed correctly, if the basi-occipital 

 were likewise divided, and separated along the median line, of which I know not 

 any example. 



