438 CETACEA. 



of the teeth having been worn to flat or ;rouncled surfaces, having the same lateral 

 and longitudinal dimensions as the enlarged bases. 



At last the deposit of bone around the bases of the teeth proceeds to such an 

 pxtent that the sockets disappear, and the teeth, if left in the month, are only in the 

 fleshy gums ready to drop out. Erom these facts it is evident that important changes 

 take place in the bases of the teeth after the early closiu-e of the pnlp cavity. In 

 many of the teeth the bases are tln-own into strongly marked ridges, which at their 

 free ends may resemble so many short irregular fangs (see woodcut, fig. 13), which, 

 as Professor Owen has observed, is an exceptional character in the existing car- 

 nivorous Cetacea. 



In studying the foregoing changes in dentition, I had recourse to microscopic 

 sections to substantiate in the development of the tissues that which was to a certain 

 extent even jiatent to ordinary vision. 



The earliest stage examined was that of the foetus, the same figured in the 

 womb (PI. XXXI). In this the teeth were not erupted, but the alveolar groove 

 was almost open and the denticles easily felt by the finger. As I have noted later on, 

 in the description of the exterior parts of this young animal, and which is shown in 

 the above figure, a series of skin ridges on the outer labial margins corresponded 

 outwardly to the non-erupted future teeth. The section, then, which I shall 

 proceed to describe is taken transversely to one of these ridges, and is drawn on an 

 enlarged scale (PL XXXVI, fig. 4). The fleshy gum on either side of the nascent 

 tooth is of the dense fibroid character. A thin film of cylindrical epithelium lines the 

 interior of the dental cavity, particularly its upper portion, or where it is least injured 

 by the razor in the process of cutting. The tooth itself is of a sj)ear-headed out- 

 line, the base on either side having a deep cleft, which, however, leaves a broad 

 pedicle of attachment. The summit of the tooth bears a thick cap of enamel 

 wliich thins downwards but reaches the very base, and even a thin lamina of 

 enamel turns round the corner or angle of the lateral basal cleft. Calcifica- 

 tion has not been completed in the rest of the tooth, but round the edges dentinal 

 cells are discernible, and more internally the future lines of the tubuli can be dis- 

 tinguished, besides the larger vascular channels of the dentinal pulp. 



No trace of cement or osteodentine is appreciable within the limits of the 

 tooth substance proper. Below the root of the tooth, however, and in continuity 

 with it, there are large open spaces, vascular and otherwise, and beyond and all 

 around open spongy bone texture. The latter tissue is barely ossified, though large 

 Haversian canals in process of formation are abundantly evident. 



At a further advanced but still youthful stage, when the teeth are thoroughly 

 calcified and erupted (PI. XXXVI, fig. 5), not only the apex but the body of the 

 tooth has a considerable coating of enamel. The dentine then composes the mass 

 of the tooth, a modicum of cement occupying the lower portion of the fang. 



In a perpendicular section of the adult tooth of Tlcdanista, taken from the 

 middle of the lower jaw (fig. 6), another stage in the chain of dental growth is met 

 with; not only is the shape of the tooth altered, but the proportions of the 



