ANATOMY OF VERTEBRATES. 141 



the ' drum of the ear ' in air-breathing vertebrates, is short, strong, 

 and immovably wedged, in the Crocodilia, between the paroccipital, 

 4, mastoid, 8, postfrontal, 12, and squamosal, 27 ; and the conditions 

 of this fixation of the pleurapophysis are exemplified in the great 

 developement of the hasmapophysis (mandible), which is here 

 unusually long, supports numerous teeth, and requires, therefore, 

 a firm point of suspension, in the violent actions to which the jaws 

 are put in retaining and overcoming the struggles of a powerful 

 living prey. The movable articulation between the tympanic, 28, 

 and the rest of the haemal arch is analogous to that which we find 

 between the thoracic pleurapophysis and hamiapophysis in birds. 

 But the hamiapophysis of the mandibular arch in the Crocodiles 

 is subdivided into several pieces, in order to combine the greatest 

 elasticity and strength with a not excessive weight of bone. The 

 different pieces of this adaptively subdivided element have received 

 definite names. That numbered 29, fig. 93, which offers the articular 

 concavity to the convex condyle of the tympanic, 28, is called the 

 ' articular ' piece ; that beneath it, 30, which developes the angle 

 of the jaw, when this projects, is the ' angular ' piece; the piece 

 above, 29, and e, fig. 95, is the ' surangular ;' the thin, broad, flat 

 piece, 31, fig. 93, applied, like a splint, to the inner side of the 

 other parts of the mandible, is the ' splenial ; ' the small accessory 

 ossicle, 3i', is the 'coronoid,' because it developes the process, so 

 called, in lizards ; the anterior piece, 32, which supports the teeth, 

 is called the ' dentary.' The purport of this subdivision of the 

 lower jaw-bone has been well explained by Conybeare 1 and 

 Buckland, 2 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 snapping 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 

 splicing and bracing together of thin flat bones of unequal length 

 and of varying thickness, affords compensation for the weakness 

 and risk of fracture that would otherwise have attended the 

 elongation of the parts. In the abdomen of the crocodile the 

 analogous subdivision of the hamiapophyses, there called abdo- 

 minal ribs, allows of a slight change of their length, in the 

 expansion and contraction of the walls of that cavity ; and since 



1 ' Geol. Trans.' 1821, p. 565. 2 ' Bridgewater Treatise,' 1836, vol. i. p. 176. 



