THE SKULL OF OSSEOUS FISHES. 



93 



bone of Anthropotomy. All the parts of the occipital vertebra 

 are developed from or ossified in the pre-existing cartilaginous 

 cranium. 



The second ring of bones, or that which encircles the mesencepha- 

 lon, includes the ' basi-sphenoid,' the 'ali -sphenoids,' the ' parietals,' 

 and the 'mastoids' (Jiff. '60. u. & Jir/. 32.). The basi-sphenoid 

 (centrum of mesencephalic vertebra, ib. s), is always connate 

 with the pre-sphenoid, (ib. 9), forming with it a long subtriedal 



bone (basi-pre-sphenoid*), usually 

 bifurcate posteriorly, and mox-e or 

 less expanded beneath the cranial 

 cavity ; it is then continued for- 

 wards (sometimes after sending out 

 a pair of lateral processes, as in the 

 Perch, more commonly without such 

 processes) along the base of the in- 

 ter'Orbital space to near the fore-part 

 of the roof of the mouth : its pos- 

 terior extremity is joined by a squa- 

 mous suture, as in Diodon, to the basi-occipital ; or more commonly, 

 as in the Cod, is firmly wedged by a kind of double gomphosis into 

 the basi-occipital : its expanded part supports the petrosals and ali- 

 sphenoids : the pre-sphenoidal prolongation (9) articulates with the or- 

 bito-sphenoids and the ethmoid, when this is ossified, and it terminates 

 forwards by a cavity receiving the pointed end of the vomei'. It is 

 this portion of the basi-pre-sphenoid which manifests the loss of sym- 

 metry in the flat fishes (Pleuroncctida;), being twisted up to one 

 side of the skull. The basi-pre-sphenoid varies in form with that of 

 the head in general, being longest and narrowest in long and narrow 

 skulls, and the converse ; the whole of its upper surface is commonly 

 rough for articulation with the petrosals and ali sphenoids ; rarely does 

 any portion enter into the direct forniation of the cranial cavity, and 

 then, perhaps, e. g. in the Cod, a small surface may support the pituitary 

 sac. When it enters more largely into the formation of the floor of 



Disarticulated neural arch of parietal ver- 

 tebra, viewed from behind: Gadus Morrhua. 



* The ossification of the basi-pre-sphenoid jirocceds from a common centre ; but 

 this does not invalidate its general homology with the bodies of the second and 

 third cranial vertebra?, as manifested by their neurapophyses (alisphenoids and orbito- 

 sphenoids) and spines (parietal and frontal), any more than tlie ossification from a 

 single centre of the common snp[)()rting bifurcate bono of the neural and ha;mal 

 spines ol the caudal fin disproves the inference that that single bone re])rescnts the 

 coalesced bodies of the terminal vertebrx to which those spines belong. The par- 

 tially united radius and ulna of the frog are ossified from a common centre at tlieir 

 coalesced proximal ends. 



