70 ILUNOIS BIOLOGICAL MONOGRAPHS [70 



examples — the vomer bears teeth and some have thought that these were an 

 integral part of the bone. Schleip (1903) has shown that in Salmo the vomer 

 develops, as in Amiurus, from deep-lying connective tissue beneath the chon- 

 drocraniimi and that the teeth arise independently. In some of the Characini- 

 dae and Cyprinidae, the vomer is intimately connected with the cartilage of 

 the ethmoid plate. In all adult teleosts it is unpaired, although it may arise 

 from paired parts as in Esox (Walther, 1882). In some it forms a cap on the 

 anterior end of the ethmoid cartilage. In none of the forms described up to 

 this time has a condition wholly similar to that in Amiurus been found. Its 

 limitation to the ventral surface of the cranium is not the common type of 

 development, as there are usually anterior or lateral processes projecting for 

 articulation with the cranial bones of the dorsal surface. 



The orhito sphenoid. This is a large unpaired bone forming the floor and 

 side walls of the cranium between the orbital and optic foramina (Figs. 6, 7, 

 16, 20). It is visible externally in the wall of the orbit, overlapped dorsally 

 by the frontal and interdigitating anteriorly with the ectethmoid in both the 

 upper and lower margins of the orbital foramen. This part of the bone is 

 only a thin lamella on the cartilage which persists in the side walls of the cra- 

 nium and unites with a similar lamella on the internal svu^ace of the cartilage in 

 the margins of the foramen. 



The anterior end of the alisphenoid bone extends down in front of the optic 

 nerve so that the orbitosphenoid is limited to the ventro-anterior wall of the 

 foramen. In the midventral margin of the foramen it interdigitates externally 

 with a lateral process of the parasphenoid, and the line between them extends 

 across the ventral surface of the cranium to the optic foramen of the other side. 

 The immediate middle part of this interdigitation is visible only upon removal 

 of the vomer and the anterior spicules of the parasphenoid (Fig. 16). The 

 ventral anterior end of the orbitosphenoid is separated from the laterally lying 

 ectethmoids by cartilage. The median anterior margin lies quite far posterior 

 to the main part of the supraethmoid, but several spicules from the latter 

 bone extend posteriorly to it along the dorsal surface of the parasphenoid 

 (Figs. 7, 16). The interval between the main parts of the supraethmoid and 

 the orbitosphenoid is occupied by the ethmoid cartilage. 



There is a notch on each side of the bone in the orbital wall, just behind the 

 orbital foramen, for the attachment of the pterygoid muscles (Fig. 20). The 

 ventral wall of the notch continues posteriorly as far as the optic foramen and 

 forms a shelf supporting the optic nerve. The whole ventral surface of the 

 bone is roughened by fine lines. On the median ventral part of the bone, which 

 is closely applied to the dorsal surface of the parasphenoid, the Hnes run 

 longitudinally, and on the lateral parts in the orbital walls, they radiate from 

 a center on each side (Fig. 16). 



A longitudinal section through the cranimn shows the relative extent and 

 thickness of the orbitosphenoid (Fig. 7). The dorsal posterior surface of the 



