112 LECTURE V. 



posterior margin of the hypo-tymjaanic is grooved for the reception of 

 the part of the pre-opercular (34), its inner side is excavated for the 

 insertion of the pointed end of the meso-tympanic (2g), and the 

 anterior angle is wedged between the pre-tympanic (27) 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 Sjjcirus insidiator, or Sly-bream {Epibuhis, Cuv.), 

 presents the most remarkable modification of the hypo-tympanic 

 {Jiff. 37. 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 meso-tympanic. Thus, 

 in the extensive protractile and retractile movements of the mouth, 

 the under jaw swings backwards and forwards 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 pre-oper- 

 culum to its angular piece {^g. 37. /, 30). 



By the confluence of the meso-tympanic with the epi-tympanic, and 

 of the pre-tympanic with the hypo-tympanic, in the Eel tribe, the sus- 

 pensory pedicle of the lower jaw is reduced to two pieces, as it is in 

 the 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 Murcsnce, viz. the confluent 

 pre-tympanic and hypo-tympanic pieces. This progressive simplifi- 

 cation, 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 squamo-temporal bone 

 and the malar bone of higher animals, but also the 'symplectic' — a 

 peculiar ichthyic bone — are superadded to the 'tympanic' or quad- 

 rate bone of Reptiles and Birds, in the formation of the suspensory 

 pedicle of the under jaw of Fishes. Ascending to the higher gene- 

 ralisations of homology, we see in the tympanic pedicle a serial re- 

 petition of the palatine bone ; and, in both, the ribs or pleurapophyses 

 of contiguous vertebra3 specially modified for the masticatory func- 

 tions of the arches they support.* 



The mandible, or lower jaw (ha^mapophysis of the frontal ver- 

 tebra {fig. 30. 29, 32), is the loAver portion of the arch, being arti- 

 culated to the hypo-tympanics above, and closed by a ligamentous 

 union or bony symphysis with its fellow at its lower end. The term 



* The division of the pleiirapophysis of the frontal vertebra into four tympanic 

 pieces no more destroys its individuality than does the divisim of the maxillary bone 

 in Lepidosteus the individuality of that bone. 



