122 ANATOMY OF VERTEBKATES. 



trochlea; and by its anterior border to the pretympanic. ib. b. The 

 mesotympanic is confluent with the epitympanic in the Siluroid, the 

 Mursenoid, and some other fishes ; but does not join the epitympanic 

 in the Lepidosteus, being in that fish supported by the preopercular. 



The pretympanic, figs. 81, 27, 84, Z, is an oblong bony scale, 

 with the posterior margin thickened and grooved for the reception 

 of the fore part of the mesotympanic and the upper and fore part 

 of the hypotympanic. It is confluent with the hypotympanic in 

 the Conger and Mursena : it does not join either this or the meso- 

 tympanic in the Lepidosteus. 



The hypotympanic, figs. 81, 28, 75 and 84, 2$d, is a triangular 

 plate of bone, like the epitympanic reversed, bearing the articular 

 convex trochlea for the lower jaw upon its inferior apex and with 

 a straight base. The posterior margin of the hypotympanic is 

 grooved for the reception of part of the preopercular, ib. 34, 

 its inner side is excavated for the insertion of the pointed end 

 of the mesotympanic, and the anterior angle is wedged between 

 the pretympanic and the pterygoid, 24, and is firmly united to 

 the latter ; the trochlea is slightly concave transversely, convex 

 in a greater degree from before backwards. The Sly-bream 

 (Epibulus, Cuv,), presents the most remarkable modification of 

 the hypotympanic, fig. 87, 28 ; it is much elongated and slender, 

 carrying the lower jaw at an unusual distance from the base of the 

 skull, and it is itself movably connected at its upper end with the 

 mesotympanic. Thus, in the extensive protractile and retractile 

 movements of the mouth, the under jaw swings backward and for- 

 ward on its long pedicle, as on a pendulum ; the lower jaw being 

 further supported or steadied in those movements by a long ligament, 

 extending from the preoperculum to its angular piece, ib. /, so. 



By the confluence of the meso- and epi-tympanics, and of the 

 pre- and hypo-tympanics, in the Eel tribe, the suspensory pedicle 

 of the lower jaw is reduced to two pieces, as in Batrachia. In 

 the Lepidosiren it is represented, as we have seen, by a single 

 osseous piece ; but this I regard as the homologue of only the 

 lower half of the pedicle in the MurcencB) viz. the confluent pre- 

 and hypo-tympanic pieces. This progressive simplification, or 

 diminution of the multiplied centres of ossification of the tympanic 

 pedicle of Fishes, even within the limits of the class, has mainly 

 weighed with me in rejecting the Cuvierian view of its special 

 homologies ; according to which, not only the squamotemporal 

 bone and the malar bone of higher animals, but also the ( syrnplectic ' 

 a peculiar ichthyic bone --are superadded to the 'tympanic' 

 or quadrate bone of Reptiles and Birds, in the formation of the 



