772 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [2 6 J 



ticus) as well as the fact that fenestrations in the skeleton in general 

 are predisposed to proceed fi?om the peripheries of the nerve foramina; 

 as examples of which I would invite attention to the various foraminal 

 perforations that occur at the points of exit of the cranial nerves in Se- 

 lachians. 



It is my wish now to make especial mention of certain important 

 differences that exist between the labyrinth in Amia and that cavity 

 in the Teleosteans. The more complete development of the labyrinth in 

 osseous fishes has finally led to the belief that the still distinctly marked 

 elevations that bound the labyrinth niches in Amia, where they occur 

 in a rudimentary condition or are altogether absent, have resulted in a 

 mergence of thMiavity of the vestibule into the general cavity of the 

 brain case, and that the labyrinth has really moved further backwards 

 from its original position, appropriating parts that belonged to the 

 occipital region, for its concealment. Besides, in the Teleostei the an- 

 terior arch has through a reduction in size of the broad cartilaginous 

 strips, which in Amia separates it from the skull cavity, very frequently 

 come to lie in the latter. 



Finally, an important difference is seen in the fact that the almost 

 entirely cartilaginous border of the labyrinth has in the Teleosteans 

 been replaced for the greater part by a bony one. Underneath and 

 behind the foramen for the facial, the petrosal throws off a horizontal 

 lamella of bone, which in the middle line joins with the corresponding 

 lamella of the opposite side, and forms the roof of a part of the cavum 

 cranii that is closed posteriorly. It is the hindmost of the osseous 

 part of the recess for the eye muscles, which is largely membranous in 

 Amia, and of which an accurate description will be given further on. 



While the limits of the separate regions of the skull are but feebly 

 defined upon the skull-cover, quite a sharp definition takes place be- 

 tween the labyrinth and the orbital regions in the interior of the skull 

 on its cover ; this is through the means of a feebly-marked ledge, ex- 

 tending from one postorbital process to the other, and directed down- 

 wards towards the cavum cranii ; here its lower edge meets the ascend- 

 ing epiphysis coming from below. This epiphyseal ledge of the skull- 

 cover is constantly found in all Teleosteans, and represents in some 

 individual cases the only remaining portion of the original cover of the 

 primoidal skull. 



The question which considers the channels through which the sound- 

 waves of the surrounding medium reach the labyrinth in fishes has 

 never, up to the present, been the subject of an exhaustive discussion. 

 And yet the question deserves to be investigated, because quite a 

 number of peculiar formations upon the skulls of fishes will become 

 intelligible only after we have become acquainted with the nature of 

 the sound-conducting channels. It does not demand any particular 

 mention — inasmuch as an experiment is naturally out of the question — 

 that the solution of this matter can only be brought about by critical 



