428 Mr. H. Day on the 



It certainly seems remarkable that, although the para- 

 sphenoid is regarded as a disappearing bone in Re.ptilia, 

 vet it is replaced by a bone — the basisphenoid — which is 

 practically identical with the parasphenoid of Fishes and 

 Amphibia, as applied especially to fossil representatives of 

 these groups. This would lead one to expect a fusion of 

 the basisphenoid with the parasphenoid, rather than a 

 degeneration of the latter bone. 



Now this fusion can actually be traced in the development 

 of reptiles, e. g. the primitive Sphenodon, snakes, &c. 



Swinnerton and Howes (3) indicate that in Sphenodon a 

 fusion of basisphenoid and parasphenoid takes place to form 

 one bone, which in the adult resembles the " basisphenoid " 

 of snakes. At their development stage T the parasphenoidal 

 splint is still independent of the overlying pre-sphenoidal 

 cartilage, but presumably this latter is finally completely 

 underlaid or even embraced by extension of the parasphenoidal 

 shaft. 



Parker (4) shows that in the common snake the para- 

 sphenoid arises as " a long styloid tract of granular tissue 

 wedged between the trabecular and reaches the pituitary 

 body by its broad hinder end," i. e. this tract is situated in 

 the fore-half of the basis cranii, and later becomes a sharp 

 style of bone which broadens out under the pituitary space. 

 At Parker's Stage 4 the basisphenoid is seen to arise as 

 " paired symmetrical films of bone, on the outer margin of 

 the trabecular where the latter pass into the basal plate." 

 At Stage 5 these films " have grown inwards to meet and 

 form one large bone, the basisphenoid proper." It occupies 

 the roots of the trabecular, but stops in front opposite the 

 end of the parasphenoid. At Stage 8 " the basisphenoid is 

 now a compound bone, it was made from the two proper 

 basisphenoid centres and has gained a bulging floor and a 

 large carinate rostrum from the parasphenoid. But neither 

 the parasphenoid nor the basisphenoid directly floor the 

 cranial cavity." In this case there is no pre-sphenoidal 

 cartilage. 



In the above-quoted cases, therefore, it is evident that, in 

 the basisphenoid, we are dealing with a compound para- 

 basisphenoid. 



Dealing now with the fossil Reptilia, we find that the 

 parasphenoid is best developed in the Plesiosaurs, where 

 it extends forwards from the basisphenoid between the 

 pterygoids, and, as usual, the parasphenoid partly underlies 

 the basisphenoid to which it is adherent. 



In the Squamata, Ichthyosauria, and most early reptiles, 



