Parasphenoi'd of a Palceoniscoid. 4:27 



described from A, but this specimen is interesting in giving 

 us an excellent ca^t o£ the complete pituitary region, appear- 

 ing as a well-defined boss, surrounded by a sharp depressed 

 ring, ^. e. tlie reverse of the actual features. Immediately in 

 front of the pituitary region tlie cast exhibits a well-defined, 

 symmetrically disposed, shallow V-shaped line, which may 

 possibly represent the line of junction, on the dorsal surface, 

 of the ])ara- and basi-sphenoids. The anterior pair of lateral 

 processes is only fairly well preserved in impression, but thi 

 posterior pair is much more perfectly cast, the extremity o 

 the right-hand process being accurately shown, a feature no' 

 definitely shown in specimen B. 



Having described the three specimens, it now remains to 

 consider their homologies. Are they simply " parasphenoids " 

 in the restricted sense of the term — as applied to the so-called 

 " membrane '^ bone of that name — or do they represent a 

 fusion of membranous parasphenoid elements with one or 

 more cartilaginous elements of the basis cranii ? 



This question is best approached by comparing the present 

 specimens with the structures to be observed in other fishes, 

 and also in the Amphibia and Reptilia. 



When we make such a comparison, we find that the con- 

 tours of the above-described, specimens are more or less 

 completely imitated by those of the corresponding bone of 

 certain Fishes, Amphibians, and Reptiles. All three groups 

 possess a bone on the ventral surface of the cranium, 

 having anteriorly a narrow pointed shaft, which expands 

 posteriorly to form a more or less square plate, which 

 possesses usually two pairs of lateral processes. 



In living fishes and Amphibia this is a definite, very well- 

 defined membrane-bone, and there is no distinct ossified 

 basisphenoid, although rudimentary ossified structures have 

 been found in several Teleosts, and in Amm only amongst 

 the Ganoids, indicating the presence of paired basisphenoids. 



In living reptiles the bone which has an exactly similar 

 configuration to the " parasphenoid " of the lower groups is 

 termed the basisphenoid. The general statement made is 

 " that the parasphenoid bone, so characteristic of the Amphibia, 

 is for the most part very much reduced in reptiles or it may 

 be entirely wanting." Now there are only two possible 

 solutions to fit the case in reptiles, both of which may be 

 correct, however. One is that the parasphenoid is a de- 

 generating structure, as the above general statement might 

 lead one to expect ; the second, that the parasphenoid may 

 have fused with the basisphenoid. 



