ANATOMY OF VERTEBRATES. 121 



the upper jaw ; just as the bony appendages of one costal arch in 

 Birds associate its movements with those of the next. 



Tympano-manclibular arch, fig. 81, H, 25 — 32. — This presents 

 its true inverted or haemal character ; its apex or key-stone formed 

 by the symphysial junction of the lower jaw hanging downwards 

 freely, below the vertebral axis of the skull. The piers, or points of 

 suspension, of the arch, are formed by the epitympanics : each epi- 

 tympanic is articulated to both the postfrontal, 12, and the mastoid, 

 8, and is divided artificially in fig. 81; its articular surface is formed 

 in the Cod by a single elongated condyle, fig. 75, 28 ; in many 

 other fishes by a double condyle, one for each of the above-named 

 cranial parapophyses, fig. 84, 28. In the Diodon the upper border 

 of the epitympanic is articulated by a deeply indented suture to 

 the frontal, the postfrontal and mastoid bones : its posterior 

 margin supports, as in many other fishes, a circular articular sur- 

 face for the opercular bone, fig. 84, 35. Below the condyle, the 

 epitympanic in the Cod, fig. 75, becomes compressed laterally, 

 but is much expanded from before backward. The almost con- 

 stant bifurcation of both ends of the epitympanic in osseous 

 fishes, for articulation with two cranial parapophyses above, and 

 suspending two inverted arches below, make it appear like a 

 coalescence of the uppermost pieces of both those arches. In 

 most fishes the lower end is bifid, and supports both the man- 

 dibular and the hyoidean arches; the stylohyoid, fig. 81,38, being 

 attached near the junction of the epitympanic with the meso- 

 tympanic. The contiguous ribs of the Chelonia are immovably 

 connected together to ensure fixity and strength to the carapace : 

 the bulky apparatus suspended from the parietal and frontal ver- 

 tebras of osseous fishes demanded the additional strength in the 

 supporting axis which is gained by the confluence of their bodies, 

 and also by that of the proximal pieces of the pleurapophyses by 

 which the two haemal arches are suspended from those vertebrae. 

 The anterior division of the epitympanic piece articulates with 

 the preopercular, fig. 75, 34, the mesotympanic, fig. 81, 26, and 

 pretympanic, ib. 27 ; the posterior division is again bifurcate in 

 the Cod, supporting part of the preopercular and part of the 

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

 this and many other fishes. The epitympanic is simple at both 

 ends in the Carp tribe. 



The mesotympanic, figs. 81, 26, 84, 38, is a slender, compressed, 

 slightly curved, elongated bone, articulated by its upper part or 

 base to the epitympanic and preopercular ; by its lower end to the 

 inner side of the hypotympanic, reaching almost to the mandibular 



