ANATOMY OF VERTEBRATES. 109 



The basisphenoid (figs. 78 and 84, 5) is usually bifurcate poste- 

 riorly, and more or less expanded beneath the cranial cavity ; it 

 is then continued forward (sometimes after sending out a pair of 

 lateral processes, as in the Perch, more commonly without such 

 processes) along the base of the interorbital space to near the 

 fore part of the roof of the mouth : its posterior extremity is 

 joined by a squamose suture, as in Diodon, to the basioccipital ; 

 or, more commonly, as in the Cod, is firmly wedged by a kind of 

 double gomphosis into the basioccipital ; its expanded part sup- 

 ports the petrosals and alisphenoids : the presphenoidal prolonga- 

 tion (figs. 83 and 84, 9) articulates with the orbitosphenoids and 

 the ethmoid, 18, when this is ossified ; and it terminates forward 

 by a cavity receiving the pointed end of the vomer, fig. 84, 13. 

 It is this portion of the basi-pre-sphenoid which manifests the loss 

 of symmetry in the flat fishes (Pleuronectidce), 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 alisphenoids ; rarely does any portion enter into the direct 

 formation of the cranial cavity, and then, 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 the cranial cavity, it 

 usually sends upward a little process on each side ; or, as in Fis- 

 tularia, a transverse ridge. The basisphenoid is smooth below, 

 where it is usually flattened or convex, but sometimes is pro- 

 duced downward in the form of a median ridge, and sometimes is 

 perforated for the lodgment of certain muscles of the eyeball. 

 In the Polypterus both ali- and orbito-sphenoids are anchylosed to 

 the basi-pre-sphenoid, and the result is a bone that answers to the 

 major part of the ' os sphenoides' of Anthropotomy. As two large 

 and important haemal arches of the head are suspended from the 

 parapophyses of the second and third cranial vertebras, this seems 

 to be the condition of the fixation and coalescence of the bodies of 

 those vertebras in all Fishes. 



In some, e. g. Perch and Carp, the base of each alisphenoid 

 rises above the basisphenoid, and then sends inward a horizontal 

 plate, which, meeting that of the opposite alisphenoid, forms the 

 immediate support of the mesencephalon, and at the same time the 

 roof of a canal, excavated in the basisphenoid, and which 

 traverses the base of the skull, below the cranial cavity, from 

 before backwards, opening behind at the under part of the basi- 

 occipital ; this subcranial canal exists in the Salmonoids, Sparoids, 



