4 ALLIS. [VOL. I. 



In teleosts, in marked distinction to Amia, the basisphenoid 

 is a single median bone. It is, moreover, a most inconstant 

 element of the teleostean skull. According to Vrolik (No. 32, 

 p. 276) it may be found in connection with an eye-muscle 

 canal, or where that canal does not exist; there may be an eye- 

 muscle canal and no basisphenoid, or neither eye-muscle canal 

 nor basisphenoid; the bone may, in some fishes, be replaced 

 by a ligament; and in still others it may be fused with the 

 parasphenoid, and appear as a median, vertical process of that 

 bone. It is said by Vrolik (No. 32, p. 254) to develop in 

 Salmo as two lateral T-shaped nodules, which later fuse 

 to form the single impair bone. 



In all descriptions that I find of it, it lies between the ex- 

 treme hind ends of the orbits and gives attachment and support 

 to, or forms part of, the hind edge of a more or less developed 

 membranous, ventral portion of the interorbital wall or septum. 

 It may be preformed in cartilage (No. 32, p. 247), or it may 

 develop wholly or in part in that membranous part of the 

 interorbital septum to which it is destined later to give support 

 (No. 32, pp. 254, 259, and No. 20, p. 139). In this latter 

 case, in the single instance cited, the salmon, both Vrolik and 

 Parker say that the interorbital septum itself is at first formed 

 entirely of membrane, which later becomes invaded by cartilage. 



The membranous portions of the interorbital region of the 

 skull of Salmo, and hence of other fishes also, thus seem 

 to represent simply an arrested condition in the development of 

 the side walls of the cartilaginous cranium. Such being the 

 case, the teleostean basisphenoid and orbitosphenoid, both of 

 which develop either in the interorbital membrane or in the 

 cartilage which may replace it, must be of primary, and 

 not of secondary origin; and they would not necessarily 

 cease to be strictly homologous with the corresponding bones 

 in higher vertebrates, as Vrolik definitely says they do (No. 32, 

 p. 259), simply because of their histologically somewhat differ- 

 ent development in certain fishes. 



In Amia the so-called basisphenoids are described by Bridge 

 (No. 7, p. 613) as "two small osseous nodules," one on each 

 side of the head, " resting upon and in part imbedded in the 



