FISHES. 23 



affixed, consisting of two flat bony pieces, that mount beneath the 

 lower jaw, and are connected by means of a small styliform bone to 

 the inner surface of the uppermost bone of the mandibular arch 

 {ejjitym'panicum) , which connects the lower jaw to the cranium. A 

 laterally compressed unpaired bone runs backwards from the union 

 of tlie two horns of the tongue-bone, and is usually connected to 

 the belt of the clavicles which sustains the pectoral fins. Beneath 

 the horns of the tongue-bone is the so-named branchiostegous 

 membrane (see above, p. 12), which completes the branchial aper- 

 tm*e ; this membrane is supported by long ossicles bent backwards, 

 called rays of the branchiostegous membrane, of which the number 

 varies in different fishes, by which the generic characters may be 

 compared; many fishes have seven such rays, the Cyprini only 

 three, where however they are broader. 



The three bony plates situated behind the prceojyerculum may 

 be regarded as radiations from that bone con-esponding to the bran- 

 chiostegous rays from the horns of the tongue-bone. These plates 

 form the gill-cover, the most external part of the bony apparatus 

 for respiration. The largest of these pieces {operculum Cuv.) is a 

 lamina attached to the ejntympanicum, and forms the upper and 

 outer edge of the branchial aperture. It has an irregular, more or 

 less triangular, form ; beneath it lies another small bony plate [sub- 

 operculum Cuv.), and a smaller is placed in front of this below the 

 jprceoperculum [interoperculum Cuv.). In the sharks, divided, carti- 

 laginous radiations, like fingers, at the posterior margin of the os 

 tympamcum, are met with, corresponding with the gill-cover; below 

 they are repeated by similar appendages of the tongue-bone, which 

 represent the branchiostegous rays. The sturgeons have no bran- 

 chiostegous rays, but they have gill-covers like other fishes ^ 



After this concise description of the skeleton, which in verte- 

 brate animals must form the commencement of all anatomical sur- 

 vey, we may proceed to treat of the remaining internal parts of 

 fishes. 



The teeth, whose insertion upon various bones we have already 

 indicated above, usually serve rather for seizing the prey and holding 



^ This subject is excellently treated by H. Rathke in Anatomisch-philosophische 

 Untersuchungen ilhcr den Kiemenapparat und das Zungenhein der Wirhelthiere. Riga 

 u. Dorpat, 1832, 4to. 



