106 BRITISH FOSSIL REPTILES. 



'Osscmen's Fossiles,' torn, v, pt. ii, pi. iv, figs. 19 J, B, C, D. The 'calcaneum' inter- 

 venes between the fibula and the ossicle supporting the two outer toes ; it has a short 

 but strong posterior tuberosity. 



The ossicle referred to represents the bone called ' cuboid' in the human tarsus. 

 A smaller ossicle, wedged between the astralagus and the metatarsals of the second 

 and third toes is the ' ectocuneiform.' 



Four toes only are normally developed in the hind-foot of the CrocodiUa ; the fifth 

 is represented by a stunted rudiment of its metatarsal, which is articulated to the 

 cuboid and to the base of the fourth metatarsal. 



The four normal metatarsals are much longer than the corresponding metacarpals. 

 That of the first or innermost toe is the shortest and strongest ; it supports two 

 phalanges. The other three metatarsals are of nearly equal length, but progressively 

 diminish in thickness from the second to the fourth. The second metatarsal supports 

 three phalanges ; the third four ; and the fourth also has four phalanges, but does 

 not support a claw. The fifth digit is represented by a rudiment of its metatarsal in 

 the form of a flattened triangular plate of bone, attached to the outer side of the 

 cuboid, and slightly curved at its pointed and prominent end. 



The teeth. — The most readily recognisable character by which the existing 

 Crocodilians are classified and grouped in appropriate genera, are derived from 

 modifications of the dental system. 



IQ lo OO OO 



In the Caimans (genus AUifjatof) the teeth vary in number from -^ — i-^ to ^^^r — - : 



the fourth tooth of the lower jaw is received into a cavity of the alveolar surface of the 

 upper jaw, where it is concealed when the mouth is shut. In PI. 1 C, fig. 2, these 

 pits are shown behind the last premaxillary tooth e, in an eocene Alligator from 

 Hordwell. In old individuals of the existing species of Alligator, the upper jaw is 

 perforated by the large inferior teeth in question, and the fossae are converted into 

 foramina. 



In the Crocodiles (genus Crocodilus) the fourth tooth in the lower jaw is received 

 into a notch excavated in the side of the alveolar border of the upper jaw, as in fig. 1, 

 PI. 1 C, behind the tooth e, and is visible externally when the mouth is closed, as in 

 PI. ] B, fig. 1. In most Crocodiles, also, the first tooth in the lower jaw perforates 

 the premaxillary bone when the mouth is closed, as in PI. A 2, between the teeth 

 marked a and h. 



In the two preceding genera the alveolar borders of the jaw have an uneven or 

 wavy contour, and the teeth are of an unequal size. 



In the Gavials, (genus Gavialis) the teeth are nearly equal in size and similar in 

 form in both jaws, and the first as well as the fourth tooth in the lower jaw, passes 

 into a groove in the margin of the upper jaw when the mouth is closed, PI. 1. 



In the Alligators and Crocodiles the teeth are more unequal in size, and less regular 



