The tooth-germ c (figs. 80 and 81) is developed from the membrane covering the angle between the floor and the inner wall of the socket. It becomes, in this situation, completely enveloped by its capsule, and partially calcified, before the young tooth penetrates the in terior of the pulp-cavity of its predecessor.
The matrix of the young growing tooth affects, by its pressure, the inner wall of the socket, as shown in figs. 80 and 81, and forms for itself a shallow recess ; at the same time it attacks the side of the base of the contained tooth : then, gaining a more extensive attachment by its basis and increased size, it penetrates the large pulp-cavity of the previously formed tooth either by a circular or semicircular perforation.
The size of the perforation in the tooth, and of the depression in the jaw, proves them to have been in great part caused by the soft matrix, which must have produced its effect by exciting absorbent action, and not by mere mechanical force. The resistance of the wall of the pulp-cavity having been thus overcome, the growing tooth and its matrix recede from the temporary alveolar depression, and sink into the substance of the pulp contained in the cavity of the fully-formed tooth.
As the new tooth grows, the pulp of the old one is re moved ; the old tooth itself is next attacked, and the crown, being undermined by the absorption of the inner surface of its base, may be broken of by a slight external force, when the point of the new tooth is exposed.
The new tooth disembarrasses itself of the cylindrical base of its predecessor (fig. 81, a) with which it is sheathed, by maintaining the excitement of the absorbent process so long as the cement of the old fang retains any vital connection with the periosteum of the socket ; but the frail remains of the old cylinder, thus reduced, are sometimes lifted out of the socket upon the crown of the new tooth (as in fig. 81, a), when they are speedily removed by the action of the jaws. This is, how ever, the only part of the process which is immediately pro duced by violence ; an attentive observation of the more important previous stages of growth, teaches that the pressure of the growing tooth operates upon the one to be displaced only through the medium of the vital absorbent action which it has excited.
No sooner has the young tooth (fig. 80, b) penetrated the interior of the old one (fig. 80, a) than another germ c, begins to be developed from the angle between the base of the young tooth and the inner alveolar process ; or in the same relative position as that in which its predecessor began to rise, and the processes of succession and displacement are carried on uninterruptedly throughout the long life of these cold-blooded carnivorous reptiles.
From the period of exclusion from the egg, the teeth of the crocodile succeed each other in the vertical direction ; none are added from behind forwards like the true molars in Mara malia. It follows, therefore, that the number of the teeth of the crocodile is as great when it first sees the light as when it has acquired its full size ; and, owing to the rapidity of their succession, the cavity at the base of the fully-formed tooth is never consolidated.
The fossil jaws of the extinct Crocodilians demonstrate that the same law regulated the succession of the teeth at the ancient epochs when those highly-organised reptiles prevailed in greatest numbers, and under the most varied generic and specific modifications, as at the present period, when they are reduced to a single family composed of so few and slightly varied species as to have constituted in the system of a small fraction of the genus Laccrta.
The large, thick, externally ridged or pitted scales, though common to the Crocodilian order, are not peculiar to them. The labyrinthodont the thecodont Staganolepis, the lacertian Saurillus, have left similar petrified scales.
Crocodilians with cup-and-ball vertebra', like those of living species, first make their appearance in the greensand of North America (Crocodiles basifissus and C. basitruncatus). In Europe their remains are first found in the tertiary strata. Such remains from the plastic clay of Meudon have been referred to C. isorhynchus, C. ccelorhynchus, C. Becquereli. In the calcaire grossier of Argenton and Castelnaudry have been found the C. Rallinati and C. Dodunii. In the coeval eocene London clay at Sheppy Island the entire skull and charac teristic parts of the skeleton of 0. totiapicus and C. Champ sada occur. In the somewhat later eocene beds at Brackles ham occur the remains of the gavial-like C. Dironi. In the Hordle beds have been found the C. Hastingsice, with short and broad jaws ; and also a true alligator (C. Hantoniensis). It is remarkable that forms of proccelian Crocodilia, now geo graphically restricted—the gavial to Asia, and the alligator to America—should have been associated with true crocodiles, and represented by species which lived, during nearly the same geological period, in rivers flowing over what now forms the south coast of England.
Many species of procceliau Crocodilia have been founded on fossils from miocene and pliocene tertiaries. One of these, of the gavial sub-genus (C. crassidens), from the Sewalik tertiary, was of gigantic dimensions.