74 ILLINOIS BIOLOGICAL MONOGRAPHS [74 



ventral surface of the prootics (Fig. 7). The wings which extend dorso-laterally 

 between the orbitosphenoid and the prootic, are characteristic of the teleostean 

 parasphenoid. In some forms they he behind the fifth nerve and in others in 

 front of it, which Swinnerton regards as of importance in establishing the mor- 

 phology of the bone. The ridge for the insertion of the arcus palatini muscles 

 is characteristic of the ventral surface of the parasphenoid. There are usually 

 two -of these ridges as in Amiurus, but in Scomber (AUis, 1903) they have fused 

 into a single ridge along the middle line of the bone. 



Before Huxley, the parasphenoid was regarded as the homologue of the 

 mammalian basisphenoid and was called the 'sphenoide basilare.' Huxley 

 (1864) recognized the peculiar relation of this bone to the ventral surface of the 

 cranium and denied this homology because it extended beneath the anterior 

 bones of the cranium and posteriorly beneath the basioccipital, and was easily 

 detached from the chondrocranium. He limited the distribution of the para- 

 sphenoid to the branchiate vertebrates and this idea was prevalent until Sutton 

 (1884) maintained that the bone was present in the higher groups and that it 

 was the representative of the vomer of the mammals. Its great development 

 in the fishes and in the Amphibia is due to the weak base of the chondrocranium. 

 With the highly ossified condition of the sphenoidal cartilages of the Mammalia 

 the bone was no longer needed as a support of the cranium and so it became a 

 part of the septum of the nasal passages. 



The development of the parasphenoid from membrane below the anterior 

 basicranial fenestra has been known for a long time in many groups and, as 

 noted above (page 43), the development in Amiurus is typical. In some of the 

 lower teleosts, among which is Osteoglossum (Ridewood, 1904), the parasphe- 

 noid bears teeth, but it is not known whether they are primarily an integral part 

 of the developing bone or have fused with it later. 



The suprasphenoid. This bone in Amiurus is in a very unspecialized condi- 

 tion as compared with those teleosts which have an interorbital septum. It 

 lies cerebral to the parasphenoid and is firmly fused to it (Fig. 7). It occupies 

 the floor of the cavum cranii between the optic and trigeminal nerves; anteriorly 

 it overlaps the orbitosphenoid, posteriorly the prootic. Laterally, between 

 the foramina for these nerves, it interdigitates with the alisphenoid. As stated 

 above, it is developed from membranous connective tissue between the 

 trabeculae in the fenestra hypophyseos (Fig. 32). It has all the characters 

 common to the suprasphenoid bone of the other teleosts, except the eye muscle 

 relations. The homologies of this bone have been discussed earlier in the 

 paper, as the terminology used is based principally upon its developmental 

 relations. 



The alisphenoids. These are a pair of bones, one on each side of the cranium 

 between the optic and trigemino-facial foramina (Figs. 7, 20). Their ventral 

 ends are separated externally from each other by the parasphenoid, and inter- 



