NEKVOUS SYSTEM OF FISHES. 209 



The largest (^fig. 30. ig'') is an oval or round flattened body, striated 

 and indented at the margins ; convex, and sometimes grooved {Ephip- 

 pus), on one side, more or less excavated on the other. The smaller 

 otolite is less regular in its shape : there are often two of these. 

 Each semicircular canal rises by an ampulliform end ^fig. 58. e,f, g) 

 from the 'alveus communis,' (a) and communicates, by the opposite 

 end, either with another canal, or with the vestibule separately, 

 without previous dilatation : tAvo of the canals are sub-vertical in 

 their course, and are antei'ior (e) and posterior (</) in relative posi- 

 tion : the third canal {f) is external and horizontal. A septum is 

 continued across the ampulla from the line where the division of the 

 acoustic nerve enters : a large proportion of the nerve expands upon 

 the sac of the otolites. All the parts of the labyrinth are of large 

 size ; yet the compartments of the otocrane which the semicircular 

 canals traverse " are much too wide for them, and they are sujiported 

 in these passages by a very fine cellular membrane." * 



The Chimaerae and Sturgeons resemble the bony fishes in the form 

 and position of the labyrinth. The otolites are a hard chalky sub- 

 stance in the Lepidosiren ; in which fish, as well as in the Plagio- 

 stomes, the whole labyrinth is buried in the thick basi-lateral walls of 

 the ci'anium : in both the cartilaginous capsule conforms more closely 

 in size and configuration to the membranous labyrinth ; its passages 

 and compartments are lined by a delicate perichondrium, from which 

 filaments are detached to support the semicircular canals. The vesti- 

 bule is divided in the Skate and Tope into three compartments, — the 

 ' alveus communis' {fig. 59. a) ; the sac (ib. h) and the cysticule (ib. c), 

 and it has also a small caecal appendage, called the ' utricule ' (ib. d) : the 

 otolitic contents are like soft chalk, and are disposed in two masses ; one 

 very large, occupying the sac and the cysticule, the other small, and 

 lodged in the utricule. A canal extends in Sharks from the vesti- 

 bular capsule to a foramen at the upper part 

 of the occiput, which is closed by the skin. 

 In the Rays, besides this 'fenestra capsulse' " 

 (ib. v\ a membranous canal (ib. o, p) is pro- 

 duced from the vestibule itself, and, as Hunter 

 well describes, " from the union of the two 

 pei'pcndicular canals (fig. 59. p) ; which is 

 the case witli all the Ray kind, the external 

 orifice being small, and placed on the upper 

 flat surface of the head." So minute and 

 Organ o^ Hearing, Skate approximated are these 'outer ears,'t that 



♦ Hunter, vii. iii. p. 101. 



f II). p. 389. pi. xxxiii. fig. 1. Iluntor's original memoir " On the Organ of 



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