200 PISCES. 



cesses. Upon its sides are found the hard, oval auditory capsules, 

 united continuously with the skull, and enclosing the membranous 

 labyrinth ; upon the upper part of the skull itself we remark, as 

 in the Rays, a fontanelle closed by fibrous membrane. In front also' 

 and inferiorly, the arch of the cranium is to a great extent mem- 

 branous. Superiorly, and in front of the skull, is the opening of the 

 nasal tube, which perforates the palate, and on its posterior wall the 

 olfactory nerve is distributed as it issues from the cranium. A num- 

 ber of cartilaginous plates lying behind each other belong to the parts 

 of the mouth : thus there is found an anterior and posterior operculum 

 of the oral aperture, to which are superadded inferiorly a pair of 

 lateral plates ; a ring-shaped piece of cartilage, the labial ring, cor- 

 responds to the maxilla, and to this is fastened laterally a styliform 

 cartilage, which is directed horizontally backward. 



The skull is still more simple in Ammocaetes, where the parts of 

 the mouth just described are in a great measure wanting, while the 

 cranial capsule is essentially similar in structure to that of Pe- 

 tromyzon. 



In the family Myxinoides very varied and peculiar forms of oral 

 cartilages are developed, while the cranium of the genera Myxine 

 and Bdellostoma admits of a comparison with that of Petromyzon. 

 All the essential parts described above, namely, in reference to the 

 capsule of the brain, are here repeated, and we meet with similar 

 hard auditory capsules ; but there is also a singular frame-work to the 

 pharynx formed of cartilaginous hoops, a long nasal tube consist- 

 ing, like a trachea, of cartilaginous rings, the nasal capsule, and the 

 palatal plate with its raised borders. In the Myxinoides the nasal 

 canal is open inferiorly toward the palate, while in Petromyzon and 

 Ammocaetes it there terminates in a ccecal manner. 



In the remarkable genus Amphioxus s. Branchiostoma, a case or 

 shell is completely wanting for the rudimentary brain, which is 

 merely included in a membranous capsule ; in the mouth, however, 

 is found a cartilaginous ring with cartilaginous filaments ; there is 

 also present a system of oral cartilages, as in the Sharks, Chimeerae, 

 and Petromyzon. 



The structure of the head in the Amphibious Fishes, including 

 the genus Lepidosiren, of whiah it is doubtful whether it should be 

 arranged among the members of the present class or among Reptiles, 

 offers very great peculiarities. The skull, as in all Fishes, is irn- 

 moveably united to the occiput, presents a cartilaginous substratum 

 upon which the peculiarly-formed bones of the head repose, and 



