| OCR Text |
Show 1896.] MAMMALIAN DENTITION. 571 ment of the lingually-placed dental lamina would certainly suggest that they should be regarded as persistent milk-teeth; but against this view we have the fact that all the other anterior teeth (incisors, canine, and premolars) are now shown to be permanent teeth with vestigial milk predecessors, and also that in all other cases among the Placentalia where the teeth (especially the incisors) undergo great enlargement, as is the case with r^-r of the Shrew, it is invariably the permanent teeth which are enlarged, and not unfre-quently the corresponding milk-teeth are reduced and even aborted (Lepus &c). This condition is so universal that I a m inclined to believe that in the Shrew, in the case of ~ as with 1.1 the rest of the incisors &c, the milk set has been reduced, but that here this reduction has been carried further until all trace of di. 1 has been lost, this being due to the large size and earlier development of pi. 1, these latter being developed far in advance of the posterior teeth. The lingual growth of the dental lamina is comparable to that which has been observed in connection with the successional teeth in so many forms (Seal (6), D o g (24), &c), and which is there regarded as evidence of the existence of a 3rd or 4th set of teeth which might replace the permanent set, and to which the term postpermanent dentition has been applied. This structure may owe its greater development in the Shrew to the early appearance of the permanent set and to the complete loss of the milk series. The relations of these teeth may be expressed as follows, bearing in mind that the milk-dentition is functionless and probably uncalcified:- fl 2 3 f 1 (i.4) (0 2(c) 3 4 fl 2 3 T I (*) (J). n)(D- . py W (fl. MJ L< (73Iyv L-~"<\ ' '' e -^< u(v4) 'M- < 1 2 (3) [0 'v0 0 0 4 1 2 3 CENTETBS. My material for the study of this interesting form consisted of two foetal specimens of different ages, measuring respectively in total length 36 mm., head length 12 mm., and 70 m m . with ahead length of 20 mm., together with young and adult dried skulls in the teaching collection of the Boyal College of Science and the more numerous specimens in the British Museum. The relations of the milk and permanent teeth of the Tanrec are fairly well known, the most striking being the non-replacement of the 3rd upper incisor. This is especially interesting on account of what we have seen in Gymnura and Erinaceus, where that tooth is likewise only functional in one dentition ; but here the resemblance seems to stop, for in Gymnura and Erinaceus the functional third incisor undoubtedly belongs to the replacing or permanent 37* |
