CROCODILIA. 125 



In the upper jaw the fourth, ninth, and tenth are the largest ; and the fifteenth 

 and sixteenth exceed in size those immediately before and behind them. The alveolar 

 border of the jaw increases in depth to form the sockets requisite for firmly lodging 

 these larger teeth, and gives rise to the festooned outline of the jaw, which is found in 

 all Crocodiles and Alligators in proportion as the teeth are unequal in size. 



The lower jaw presents the same compound structure as that in the Crocodilia, 

 with the general form characteristic of that in the Alligators and in most of the true 

 Crocodiles : the symphysis, e. g. is as short as Crocodilus biporcatus and the Alligator 

 niger, in which it extends as far back as the interval between the fourth and fifth 

 socket. This is the relative position of the back end of the symphysis in a fine and 

 perfect under jaw of the Crocodilus Hasfingsies in the collection of the Marchioness of 

 Hastings. In a portion of the under jaw of apparently the same species of Crocodile, 

 from the same locality, in the collection of Searles Wood, Esq., F. G. S., the symphysis 

 terminates opposite the interval between the third and fourth tooth. 



The chief distinction observable between the modern Crocodiles and Alligators in 

 the lower jaw is the greater relative size of the vacuity between the angular (30) and 

 surangular (29) pieces, and the greater relative depth of the ramus at that part, in the 

 Alligators. In these characters the lower jaw of the present species more resembles 

 that of the true Crocodiles ; although, as the vacuity in question is somewhat larger, 

 a slight affinity to the Alligator might be inferred from that circumstance. The 

 comparative figures of the hinder third of the mandibular ramus in Plate 1 E, 

 figs. 4, 5, 6, will exemplify the difference in question, and the degree of proximity to 

 the crocodilian and alligatorial characters respectively. 



With regard to another character deducible from the relation of the backwardly- 

 produced angle of the jaw to the articular surface, the Crocodilus Hasfuigsice more 

 decidedly resembles the Alligator : I allude to the depth of the excavation between 

 the articular cavity (29) and the end of the angle (30), and to the lower or higher level 

 of the angle itself: the fossil jaw (fig. 5) resembles the Alligator (fig. 6) in this 

 respect more than the Crocodile (fig. 4). The alveoli are twenty in number in each 

 ramus of the Crocodilus Hastingsice : the third and fourth are large, of equal size, and 

 close together ; behind these the eleventh, twelfth, and thirteenth are the largest, and 

 the alveolar ridge is raised to support them ; after the seventeenth the summits of the 

 crowns of the teeth become obtuse, and the crowns mammilloid, and divided by a 

 constriction or neck from the fang ; they each, however, have a separate socket, as in 

 the Crocodiles, the septa not being incomplete at the hinder termination of the dental 

 series, as in the Alligator niger figured in my ' Odontography.'* 



Fig. 3, PI. 2 B, gives a representation, of the natural size, of the cranial platform 

 of a young Crocodilus Hasliiigsice in the collection of Searles Wood, Esq. ; the hemi- 

 spheric depressions in the surface of the bone are more regular, distinct, and relatively 



* Tom. ii, pi. Ixxv, fig. 3. 



