CROCODILIDAE 467 



the upper jaw, and this pit is in some adult specimens trans- 

 formed into a hole, the tip of the tooth appearing on the upper 

 surface through the perforation. Most of the other teeth of the 

 lower jaw are overlapped by those of the upper jaw. The number 

 of teeth on either side amounts to seventeen to twenty in the 

 upper and eighteen to twenty in the lower jaw. The nasal bones 

 form not only the posterior border of the nasal groove, but they 

 divide the latter by a median bony septum. The dorsal shield is 

 formed by six or eight longitudinal series of keeled bony scutes, 

 which, although standing close together, do not articulate with 

 each other. Ossification of the gular and ventral scutes is absent 

 or very slight. 



Alligators occur in the fluviatile deposits of the age of the 

 Upper Chalk in Europe, where they did not die out until the 

 Pliocene age ; they are now restricted to two species, one in the 

 Southern States of North America, the other in China. 



A. mississippiensis. The much-depressed and broadly rounded 

 snout bears some resemblance to that of a pike, hence the now 

 discarded specific name of lucius. The neck is protected by two 

 pairs of large scutes, which form a square, interrupted in the 

 middle line, with a pair of small scutes in front and another behind. 

 Of the eighteen transverse dorsal rows of scutes eight are broad 

 and prominent. The fingers are about half webbed, the outer 

 toes about two-thirds webbed. The general colour is greenish black 

 or dark brown above, yellowish below. Young specimens have 

 yellowish cross-bands on a darker brown ground. 



The Alligator's northern limit is the mouth of the river 

 Neuss in North Carolina, 35 N. lat. From this point they 

 abound near the mouths of all the creeks and rivers as far south 

 as the Rio Grande, ascending the Mississippi to the entrance of 

 the Eed Eiver in 33 50' N. lat. 



The habits and the embryology of the American Alligator 

 have been described by S. F. Clarke, 1 who gives the following 

 vivid and minute account : 



" Usually one finds them in the waters of the smaller streams 

 and ponds, lying with only the tip of the nose and the eyes 

 exposed, or lying on an exposed place on the bank where the 

 grass and other plants are beaten down, and the black, rich mud 

 of the river bank is smoothed by the repeated movements of the 

 1 J. Morphol. v. 1891, p. 181. 



