THE SKULL OF OSSEOUS FISHES. 1 1 1 



Strength to the carapace : the bulky apparatus suspended from the 

 parietal and frontal vertebras demanded the additional strength to 

 the supporting axis which is gained by the confluence of their 

 bodies, and apparently by the confluence of the proximal pieces of 

 the pleurapophyses by which the two haamal arches are suspended 

 from those vertebra?. The anterior division of the epi-tympanic 

 piece articulates with the pre-opercular (34), the meso-tympanic 

 (2g), and pre-tympanic (27) ; the posterior division is again bifur- 

 cate in the Cod, supporting part of the pre-opercular and part of the 

 opercular bone. A strong crest projects from its outer surface in 

 this and many other fishes. The epi-tympanic is simple at both ends 

 in the Carp tribe. 



The meso-tympanic {fig. 30. 26), or 'symplectic' of Cuvier, is 

 a slendei", compressed, slightly curved, elongated, triangular bone, 

 articulated by its upper part or base to the epi-tympanic and pre- 

 opercular ; by its lower end to the inner side of the hypo-tympanic, 

 reaching almost to the mandibular trochlea; and by its anterior border 

 to the pre-tympanic. The upper part of its posterior border is free, 

 and gives attachment to the membrane that fills up the vacuity 

 between it, the pre-opercular and hypo-tympanic bones. The 

 meso-tympanic is confluent with the epi-tympanic in the Siluroid, 

 the Murjenoid, and some other fishes ; but does not join the epi- 

 tympanic in the Lepidosteus, being in that fish supported by the 

 pre-opercular. 



The pre-tym'panic {fig. 30. 27), to which part of the suspensory 

 pedicle of the jaw Cuvier restricts the name 'caisse' or ' os tym- 

 panicum*,' is an oblong bony scale, with the posterior margin thick- 

 ened and grooved for the reception of the fore part of the meso- 

 tympanic and the upper and fore part of the hypo-tympanic. It is 

 confluent with the hypo-tympanic in the Conger and Mura^na : it 

 does not join either this or the meso-tympanic in the Lepidosteus. 



The hypo-tympanic {fig. 30. 2s) is a triangular plate of bone, like 

 the epi-tympanic reversed, bearing the articular convex trocldea for 

 the lower jaw upon its inferior apex, and having its upper side or 

 base more even than the opposite base of the epi-tympanic. The 



* This is perhaps one of the best examples of the extent to which Cuvier was 

 influenced by the idea or principle of liomolo<iy, when a determination liad orifrin- 

 ateil from his own comparisons; iaw of the names imposed by Geoirroy St. Ililaire, 

 in confiirmity witli his peculiar views, seem more overstrained tlian tlie transference 

 of the name and signification of the little process siipportinj!; the ear-drum in man 

 to a small segment of a strong pedicle, wholly deprived of the proper tymjianic 

 function, and with which the Iiomology of the human tymjianic process of tlic 

 tomporrd bone can only be established by taking the pedicle of the lower jaw in 

 Fishes as a whole. 



