2 ALLIS* [VOL. I. 



until they have attained some considerable size. They fuse 

 first with the orbitosphenoids; then with each other, by 

 sending a thin shell of bone across the dorsal aspect of the 

 cartilage that separates them; and then with the basisphenoid. 

 Much later they fuse with each other in the deeper parts of 

 the separating cartilage. 



According to Thane (No. 24, vol. ii, pt. i, p. 76) the pre- 

 sphenoid of man may develop from the two nuclei of Sutton, 

 or may be "an independent growth." This latter possibility 

 is perhaps of .some considerable morphological importance, as 

 will be later seen. 



The basisphenoid first appears as two nuclei in the cartilage 

 forming the floor of the pituitary fossa (No. 29, p. 580), that 

 fossa being the sella turcica of the adult (No. 24, vol. ii, pt. 



i, P- 75)- 



In Amia the prefrontals of Sagemehl's descriptions and my 



own are the lateral ethmoids, or ectethmoids of English writers, 

 the generally accepted homologues of the lateral masses of the 

 ethmoid bone of man; and the petrosals are the prootics, 

 the generally accepted homologues of the correspondingly 

 named parts of the petrous part of the temporal bone of man. 

 The sphenoidal region of the skull of Amia would accordingly 

 be, if the bones alone were considered and the homologies 

 implied in their names accepted, that part of the base and 

 sides of the skull that lies between the prefrontals anteriorly 

 and the basioccipital, squamosals, and petrosals posteriorly. 



The region so defined in Amia contains, on each side of the 

 head, four primary ossifications, three of which are called 

 by Sagemehl (No. 25) the orbitosphenoid, alisphenoid, and 

 postfrontal; while the fourth is considered by him as one half 

 of a basisphenoid bone. The postfrontal of this nomenclature 

 is the sphenotic of Bridge's descriptions of Amia (No. 7), and 

 the postorbital ossification of my own (No. i, p. 479). 



The orbitosphenoid (No. 2, Figs. 8-11) lies between the 

 orbito-nasal fenestration (No. 2) of the optic wall of the 

 skull and the optic fenestra (No. 25, p. 202); the alisphenoid 

 between the latter fenestra and the two trigeminal foramina. 

 The orbito-nasal fenestra lies, in larvae of Amia, and hence 



