226 PISCES. 



cartilaginous vestibule, and correspond to the round fenestrae ; the 

 anterior are comparable to the oval fenestrae ; between each of these 

 openings and the external skin a membranous sac is placed, which is 

 filled with a calcareous mass, and extends into the membranous ves- 

 tibule. At their commencement we find a muscle arising from the 

 integument, and serving to compress the two sacs. 



The Plagiostomi have a pair of soft calcareous concretions (oto- 

 liths), composed of carbonate of chalk, appended to the walls of the 

 sacs ; the Osseous Fishes are generally furnished with three stones 

 (lapilli), hard and dense as porcelain, and of very varied form ; one 

 of these is situated in the vestibule, two in the two chambers of the 

 sac. 



Manifold diversities occur in the form, number, and structure of 

 these otoliths. Thus the porcelainic otoliths are frequently, as in 

 Cyprinus, Gadus, Scomber, toothed at the edges, and are occasion- 

 ally, as in Sciaena, Lepidoleprus, and others, of remarkable size. 



The form, size, width, and mode of union of the semicircular 

 canals, with their position also in the cranium, exhibit likewise re- 

 markable diversities ; while, e. g., in Cobitis, they are situated quite 

 free in the cranial cavity, the external and posterior canal, or only 

 one of the two, are partly enclosed in the bones, or, as in the Pike, 

 Orthagoriscus, the Sturgeons, and Chimaeras, they are more or less 

 surrounded by cartilaginous coverings ; this, as has already been 

 mentioned, is their condition generally in the Plagiostomi. The 

 ampullae are retained in their expanded condition by peculiar double- 

 coned septa, upon which the auditory nerve expands ; the expansions 

 and terminal looped plexuses of this nerve may be very easily and 

 distinctly observed under the microscope. 



In many Osseous Fishes, but particularly the Ventrales, a most 

 remarkable communication subsists between the swimming-bladder 

 and the internal ear. The vestibule always gives off in the direction 

 backward a canal which coalesces with that of the opposite side 

 into a single reservoir (sinus impar) ; this latter is a pouch of more 

 membranous texture than the vestibule : it is situated in the basilar 

 portion of the occipital bone, bifurcates again in the direction back- 

 ward, and forms constantly a round saccule placed between the 

 first cervical vertebra and occipital bone, and filled with the fluid of 

 the labyrinth ; it is called the sinus sph&rici s. atria sinus imparis. 

 Three ossicles are placed near to the three most anterior vertebrae, 

 and are connected to their transverse processes by joints and liga- 

 ments ; they are of varied form ; the most posterior, which is the 



