436 CETACEA. 



way from the fully mature foetus of the female No. 1, except that its teeth are erupted, 

 the thirteenth tooth on the left side of the upper jaw is 0*22 inch in antero-posterior 

 breadth at its base, and 0'35 inch long, whereas in the female No. 1 and in the mother 

 of the foetus the same tooth has a basal extension of not less than 0*87 inch and a 

 length of 0*90 inch ; in other words, the tooth grows from youth to age, but the nature 

 of this growth will be seen hereafter. Eschricht described the arrangement and alter- 

 nation of the teeth, the latter feature being occasionally carried to such a degree that the 

 most anterior of the small conical teeth of the two sides, especially in the lower jaw, 

 appear as if arranged in one line. Eschricht regarded the alternation of the teeth 

 as a means seemingly to get room ; but may it not rather be due to the, so to 

 speak, natural tendency of the skull to asymmetry, because it is associated as a 

 rule with an unequal number of teeth on either side of the jaw ? Eschricht has 

 also described the general characters of the teeth in youth ; but it will be observed 

 (Table IV) that the teeth in the lower jaw are not, invariably at least, equal to 

 those in the upper, viz., twenty-nine on each side, as observed by Eschricht, because 

 in Nos. 3, 6, 12, and 14, this rule does not hold good : the teeth, however, are generally 

 more numerous in the lower than in the upper jaw. Notwithstanding the greater 

 number of teeth in the lower jaw, the two or four posterior upper teeth extend 

 behind them and have no teeth apposed to them below (see PI. XXXIX), as observed 

 by Eschricht. 



The lower are much longer than the upper teeth, and when in position in youth 

 they interlock between the latter, overlapping the sides of the upper jaw, the teeth 

 of which also, to a more limited extent, are applied at their points to the outside of 

 the mandible, and the teeth are in such close juxtaposition that the anterior teeth 

 rub against each other, and these, as Professor Owen^ has observed, retain a prehen- 

 sile structure contrary to the rule in I>elpMnid(S. 



The first tooth of the lower jaw is placed anterior to the tooth of the upper jaw. 

 The first two or three teeth are convex anteriorly and concave posteriorly, but the 

 long teeth immediately succeeding these are flattened on their anterior and posterior 

 surfaces, slightly upwardly and inwardly curved in the lower jaw, and the reverse 

 in the upper, the external surface of the teeth being convex. The points in youth 

 are very sharp. The long teeth diminish after the eighth rather rapidly. 



In the young state the alveolar groove is not divided into compartments or 

 sockets for the reception of the teeth, but is an uninterrupted furrow, from end to 

 end, in which they are lodged. The inner margins of the groove, however, are wavy, 

 and the little inward sinuosities indicate the commencement of osseous ridges, 

 which increase with age and which ultimately, and while the skull is yet young, are 

 converted by osseous growth into transverse partitions, which divide the alveolar 

 groove into a number of well-defined sockets, in each of which is lodged a tooth. In 

 the young state, the pulp cavity in the dried skull occupies the lower half of the 

 tooth, but this cavity becomes closed at an early age and while the exposed portion 

 of the tooth yet retains the characters of youth. The bases of the teeth while the 



Odont. p. 362, Cat. Ost., 1. c, p. 449. 



