1 10 LECTURE V. 



into close articulation with the palatines in the Plectognathes, and the 

 consolidation of the whole series into its normal unity is effected in 

 the Lepidosiren. The palatines form the true bases of the inverted 

 arch at their points of attachment to the prefrontals ; the intermaxil- 

 laries constitute the true apex, at their mutual junction or symphysis ; 

 the approximation of which to the anterior end of the axis of the 

 skull is rendered possible in fishes, by the absence of any air-passage 

 or nasal canal ; the pterygoids are the diverging appendages of the 

 arch * ; but are attached posteriorly to strengthen the pedicle sup- 

 porting the lower jaw, and combine its movements with those of the 

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

 associate its movements with those of the next. 



Tympano-mandihidar Arch {Jig. 30. n, iii, 25 — 32). 



This presents its true inverted or hasmal character ; its apex or 

 key-stone formed by the symphysial junction of the lower jaw hang- 

 ing downwards freely, below the vertebral axis of the skull. The 

 piers, or points of suspension of the arch, are formed by the epi-tym- 

 jKinics {fig. 30. 25), or upper pieces of the tympanic pedicles (pleura- 

 pophyses of the prosencephalic vertebra) : each epi-tympanic is 

 articulated to both the post-frontals and the mastoids, and is divided 

 artificially accordingly in fig. 30. ; its articular surface is formed in 

 the Cod by a single elongated condyle ; in many other fishes by a 

 double condyle, one for each of the above named cranial parapo- 

 physes. In the Diodon the upper border of the epi-tympanic is ar- 

 ticulated by a deeply indented suture to the frontal, the post-frontal 

 and mastoid bones ; its posterior margin supports, as in many other 

 fishes, a circular articular surface for the opercular bone. Below 

 the condyle, the epi-tympanic in the Cod becomes compressed laterally, 

 but is much expanded from before backwards. The almost constant 

 bifurcation of both ends of the epi-tympanic in osseous fishes, for ar- 

 ticulation with two cranial parapophyses above, and suspending two 

 inverted arches below, make it appear like a coalescence of the upper- 

 most pieces of both those arches. In most fishes the lower end is 

 bifid, and supports two inverted arches, the mandibular and the 

 hyoidean ; the stylo-hyoid being attached near the junction of the 

 epi-tympanic with the meso-tympanic. The contiguous ribs of the 

 Chelonia are immovably connected together to ensure fixity and 



* By Bojanus they were regarded as tlie lilis of tlie second (parietal) vcrtebm of 

 the head. (^Anatome Tesfudhiis Eurnpacc, y). 'li.) 



