ANATOMY OF VERTEBRATES. 123 



suspensory pedicle of the under jaw of Fishes. Ascending to the 

 higher generalisations of homology, we see in the tympanic pedicle 

 a serial repetition of the palatine bone ; and, in both, the ribs or 

 pleurapophyses of contiguous vertebras specially modified for the 

 masticatory functions of the arches they support. 



The mandible, figs. 81, 84, 29, 32, is the lower portion of the 

 arch, being articulated to the hypotympanics above, and closed by 

 a ligamentous union or bony symphysis with its fellow at its lower 

 end. The term ' ramus ' is aj^plied in Anthropotomy to each half 

 of the mandible, and each ramus consists of two, three, or more 

 pieces in different fishes. Most commonly it consists of two 

 pieces, one (hasmapophysis proper, 29,) articulated to the suspensory 

 pedicle, and edentulous, analogous to the maxillaiy ; and the other 

 (hrcmal spine, 32,) completing the arch, and commonly supporting 

 teeth, like the premaxillary. In the Cod, and some other fishes, 

 a third small piece is superadded, at the angle of the posterior 

 piece, fig. 75, 30. The dentary, 32, is deeply excavated, and 

 receives a cylindrical cartilage, the remnant of the embryonal 

 hamial arch, fig. 69 A, d, and the vessels and nerves of the teeth. 

 The Sudis, fig. 88, the Polypterus, and Amia, have the splint- 

 like plate along the inner 

 surface of the ramus, called 

 ' splenial : ' it supports teeth 

 and developes a coronoid pro- 

 cess. In both Sudis and Le- 

 pidosteus there is superadded a 

 small bony piece, ib. 29 a, answering to the surangular in Reptiles. 



The Diverging Appendage of the tympano-mandibular arch 

 consists of the bones which support the gill-cover, a kind of short 

 and broad fin, the movements of which regulate the passage of 

 the currents through the branchial cavity, opening and closing the 

 branchial aperture on each side of the head. The first of these 

 ' opercular ' bones is the preopercular, fig. 75, 34, which is usually 

 the longest in the vertical direction. In the Gurnards, or ' mailed- 

 cheeked' Fishes, fig. 82, the preopercular is articulated with the 

 enormously developed suborbital scale bone, 73. 



Three boues usually constitute the second series of this 

 appendage : the upper one is commonly the largest and of a 

 triangular form, thin and with radiated lines like a scale : it is 

 the opercular, figs. 75, 84, 35 : in the Cod it is principally connected 

 with the posterior margin of the preopercular, and below with the 

 subopercular, ib. 36 ; but it has usually, also, a partial attachment 

 to the outer angle of the epitympanic, fig. 84 ; and is some- 



Lower jaw, (Arapaima gigas) 



