CROCODILIA. 99 



piece (3i)j applied, like a splint, to the inner side of the other parts of the mandible, is 

 the 'splenial;' the small accessory ossicle (31') is the ' coronoid,' because it developes 

 the process so called, in lizards ; the anterior piece (32), which supports the teeth, is 

 called the ' dentary.' This latter is the homotype of the preraaxillary, or it represents 

 that bone in the mandibular arch, of which it may be regarded as the haemal spine ; 

 the other pieces are subdivisions of the hBemapophysial element. The purport of 

 this subdivision of the lower jaw-bone has been well explained by Conybeare* and 

 Buckland,t by the analogy of its structure to that adopted in binding together several 

 parallel plates of elastic wood or steel to make a crossbow, and also in setting together 

 thin plates of steel in the springs of carriages. Dr. Buckland adds, "those who 

 have witnessed the shock given to the head of a Crocodile by the act of snappino- 

 together its thin long jaws, must have seen how liable to fracture the lower jaw would 

 be, were it composed of one bone only on each side." The same reasoning applies to 

 the composite structure of the long tympanic pedicle in fishes. In each case the 

 spHcing and bracing together of thin flat bones of unequal length and of varyino- 

 thickness, affords compensation for the weakness and risk of fracture that would other- 

 wise have attended the elongation of the parts. In the abdomen of the Crocodile the 

 analogous subdivision of the hsemapophyses, there called abdominal ribs, allows of a 

 slight change of their length, in the expansion and contraction of the walls of that 

 cavity ; and since amphibious reptiles, when on land, rest the whole weight of the 

 abdomen directly upon the ground, the necessity of the modification for diminished 

 liability to fracture further appears. These analogies are important, as demonstratino- 

 that the general homology of the elements of a natural segment of the skeleton is not 

 affected or obscured by their subdivision for a special end. Now this purposive 

 modification of the hsemapophyscs of the frontal vertebra is but a repetition of that 

 which affects the same elements in the abdominal vertebrae. 



Passing next to the haemal arch of the parietal vertebra (fig. 13, H iir), we are first 

 struck by its small relative size ; its restricted functions have not required it to ^row 

 in proportion with the other arches, and it consequently retains much of its embryonic 

 dimensions. It consists of a ligamentous ' stylohyal' — its pleurapophysis, retaining the 

 same primitive histological condition which obstructs the ordinary recognition of the 

 same elements of the lumbar haemal arches. A cartilaginous ' epihyal' (39) intervenes 

 between this and the ossified ' haemapophysis' (40), which bears the special name of 

 ceratohyal. The haemal spine (41) retains its cartilaginous state, like its horaotypes in 

 the abdomen : there they get the special name of ' abdominal sternum,' here of ' basi- 

 hyal.' The basihyal has, however, coalesced with the thyrohyals, to form a broad 

 cartilaginous plate, the anterior border rising like a valve to close the fauces, and the 

 posterior angles extending beyond and sustaining the thyroid and other parts of the 



* Geol. Trans., 1821, p. 565. 



t Bridgewater Treatise, 1836, vol. i, p. 176. 



