f33] THE OSTEOLOGY OF AMI A CALVA, 779 



The i)riucipa.l difference betweeu the subcranial canal for the eye 

 muscles in Atnia and that in bony fishes, is seen in the fact that in the 

 latter it is separated from the brain case proper by an osseous partition, 

 while in Aonia this is composed only of membrane. In articles I have 

 yet to publish, it is my intention to show how this osseous partition is 

 developed in bony fishes from the neighboring bones, more particularly 

 the petrosal, by their throwing out horizontal processes that meet to 

 ossify in the median line of the skull. Commonly, too, this subcranial 

 canal extends farther back in osseous fishes than it does in Amia, even 

 to extend into the basibccipital. This results from the muscles of the 

 eye being longer in these forms, and consequently a canal of proper 

 length developes to accommodate them. 



Concerning the phylogenetic origin of the subcranial canal, G-egen- 

 baur conjectures that the canalis transversus of the Selachians is the sub- 

 cranial canal of the Teleostei, in which the muscles of the eye find lodg- 

 ment.^^ In the Selachians this canal passes from one orbit to the other, 

 obliquely through the cartilaginous basis cranii, causing the two peri- 

 orbital lymph sinuses to merge into one; in some cases it is separated 

 from the brain case by membrane only. Immediately in front of this 

 canalis transversus are found the openings for the carotids, which in 

 some forms are separated from the former also only by membrane. In 

 the orbits the recti muscles are inserted nearest to the anterior entrance 

 of the subcranial canal. Quite close to this we also find — at least in 

 several Selachians {Hexanclms) — the foramen of exit for the nervus ab- 

 ducens. 



A great deal in the structure of the parts in question, so far as ex- 

 amined in Amia, goes to support this view. Above all, the fact must 

 be noted that in Amia the canal separated from the cavum cranii is not 

 entirely devoted to the eye muscles, as in the Teleostei, but is largely 

 filled in by the lymphoid tissue. 



Now, since we have not the least ground for assuming that Amia is 

 descended from forms in which the muscles of the eye were far better 

 d^eloped, and filled the space alluded to entirely, there is but one hy- 

 pothesis possible, that Amia has in this region a preformed lymphatic 

 fossa situated at the basis cranii, into which the points of origin of the 

 recti externi only moved secondarily. But this preformed lymphatic 

 space — if we are to judge from homologous structure in inferiorly or- 

 ganized fishes — can only correspond to the canalis transversus of the 

 Selachii, which, in Amia, is remarkably widened and spread out, and 

 which has finally included the carotid canals and the surrounding nerves 

 found near the exits of these vessels. At the same time its cartilaginous 



■'2 C. Gegenbaur, Uniei'suchungen zur vergl. Anatomie d. Wirielthiere. Heft III. Das 

 Eoffskelet d. SelacMer, 1872, pag. 78. [C. Gegenbaar, Observations upon the Compara- 

 tive Anatomy of Vertebrates. Part III. The skeleton of the head in Selachii, 1872, 

 p. 78. J 



