269 



parallel to and at a little distance from the side wall of the skull. 

 This process gives attachment to the anterior end of the mass of fibrous 

 tissue that surrounds the issuing opticus, and the space between it 

 and the side wall of the skull would seem to form the little groove 

 or shelf that, in the adult, extends longitudinally forward along the 

 side wall of the skull immediately in front of the opticus foramen, 

 and supports the issuing nerve. The median plate of the bone forms 

 the floor of the cranial cavity, and is a single-layered and continuous 

 bone, the parasphenoid lying immediately beneath it. The entire bone 

 lies definitely ventral to the level of the opticus foramen, it partly 

 supports that nerve as it issues from its foramen, and it gives attach- 

 ment, either directly or through the intermediation of the mass of 

 fibrous tissue that surrounds the issuing opticus, to the recti muscles 

 of the eye ball ; these all being usual characteristics of a basisphenoid. 

 But, if this bone of Ameiurus be a basisphenoid, it would seem as if 

 the similarly named unpaired bone of all other Teleosts, wherever 

 found, must also be a basisphenoid; and this seems most improbable, 

 for in certain cases both it and a separate and independent basi- 

 sphenoid are said to be present. As a general rule, however, it may 

 be said that where there is an unpaired orbitosphenoid there is no 

 basisphenoid ; this being especially true of the Ostariophysi. 



The parasphenoid of Ameiurus is peculiar in that the base of the 

 ascending process of the bone, which begins immediately posterior to 

 the so-called orbitosphenoid, is formed of two plates which enclose 

 within them the hind end of the subopticus (trabecular?) bar of carti- 

 lage. The bone is here apparently not of perichondrial origin, but 

 the inner plate nevertheless lies internal to the cartilage of the skull 

 and there forms part of the immediate bounding wall of the cranial 

 cavity. Posterior to the hind end of the trabecular (?) cartilage there 

 is, for a few sections, a vacant space between the two plates of the 

 process of the parasphenoid, and then those plates, the inner one of 

 which gradually diminishes in height, enclose the anterior portion of 

 the prootic (parachordal ?) cartilage. It is perhaps this portion of the 

 bone of the adult that led McMurrich to conclude that the basi- 

 sphenoid was here anchylosed with the parasphenoid. 



Palais de Carnoles, Menton, France. 

 July 21st 1908. 



Literature. 



Allen, W. F., '05. The Blood- Vascular System of the Loricati, the 

 Mail-cheeked Fishes. Proc. Washington Acad. Sc, Vol. 7. 



