PROCCELIA.
The best and most readily recognizable characters by which the existing Crocodilians are grouped in appropriate genera are derived from modifications of the dental system.
In the caimans (genus Alligator) the teeth vary in number from :4: to ; the fourth tooth of the lower jaw or canine, is received into a cavity of the palatal surface of the upper jaw, where it is concealed when the mouth is shut ; in old indi viduals the upper jaw is perforated by these large inferior canines, and the fosste are con verted into foramina.
In the true crocodiles (genus Crocodiles) the first tooth in the lower jaw perforates the palatal process of the intermaxillary bone when the mouth is closed ; the fourth tooth in the lower jaw is received into a notch exca vated in the side of the alveolar border of the upper jaw, and is visible externally when the mouth is closed.
In the two preceding genera the alveolar borders of the jaws have an uneven or wavy contour, and the teeth are of unequal size.
In the gavials (genus Gavials) the teeth are nearly equal in size and similar in form in both jaws, and the first as well as the Teeth of the Gavial. fourth tooth in the lower jaw passes into a groove in the margin of the upper jaw, when the mouth is closed.
The number of teeth is always greater in the gavials than in the crocodiles or alligators. The first five pairs of teeth above are supported by the premaxillary bones ; the first, second, and fourth of the lower jaw are the longest.
The eight or nine posterior teeth are nearly conical, the rest are sub-compressed antero-posteriorly, and present a trenchant edge on the right and left side, between which a few faint longitudinal ridges traverse the basal part of the enamelled crown (fig. 80).
The position of the opposite sharp ridges, and the direction of the flat sides of the crown, are reversed in the extinct crocodile (Croc. cultridens), which in other respects most nearly resembles the gavial in the form of the teeth.
In most of the extinct species of Crocodilians the teeth are characterized by more numerous and strongly developed longi tudinal ridges upon the enamelled crown, than in the recent species ; and they are commonly longer, more slender, and sharp-pointed. But in one of the crocodiles with sub-biconcave
vertebra (Gonlopholis crassidens), from the Wealden formation and Purbeck limestone, the teeth have crowns which are as round and as thick in proportion to their length as in the recent crocodiles or alligators.
The more ancient crocodiles, from the Oolite and Lias, called Steneosauri and Teleosauri, had jaws like those of the modern gavials, but sometimes longer and more attenuated, and armed with more numerous, equal, and slender teeth, adapted for the capture of fishes, which appear to have been the only other vertebrate animals existing at those periods in numbers sufficient to yield subsistence to carnivorous marine Saurians.
In all the Teleasauri the teeth are more slender, less com pressed, and sharper pointed than in the gavial ; they are slightly recurved, and the enamelled crown is traversed by more numerous and better defined ridges—two of which, on opposite sides of the crown, are larger and more elevated than the rest. The fang is smooth, cylindrical, and always exca vatted at the base. The teeth of the S&tuosauri, or extinct crocodiles with long and slender jaws, and with vertebra: sub concave at both extremities, but with subterminal nostrils, differ from those of the Teleosaari in being somewhat thicker in proportion to their length, and larger in proportion to the jaws.
The teeth of both the existing and extinct crocodilian reptiles consist of a body of compact dentine, forming a crown covered by a coat of enamel, and a root invested by a moder ately thick layer of cement. The root slightly enlarges or maintains the same breadth to its base (fig. 80, a), which is deeply excavated by a conical pulp-cavity extending into the crown, and is commonly either perforated or notched at its concave or inner side.