1893.J ilB. M. F. WOODWABD ON MAMMALIAN DENTITION. 453 



mentary calcified tooth, which, although actually in front of the 

 former, is nevertheless morphologically and in point of origin 

 posterior to it. The 1st incisor of the adult is therefore the 

 morphological 1st incisor, and in the younger of the specimens its 

 enamel-organ only as yet is differentiated aud appears as a great 

 pear-shaped swelling of the dental lamina (enamel-ridge, " Zahn- 

 leiste"). In an older one measiu'ing 11-5 cm. there was present 

 on the inner side of the enamel-organ a slight club-shaped down- 

 growth, such as that described by Kiikenthal and others in 

 Didelphys as representing the permanent or replacing tooth 

 (" Ersatzzahn "). 



The morphological 2nd incisor is a very minute calcified rudi- 

 mentary (or rather vestigial) tooth measuring '25 mm. by "l-i mm. ; 

 its enamel-organ (fig. 2, r) is only slightly developed and appa- 

 rently, although possessing an enamel-epithelium, does not secrete 

 any enamel ; it shows a distinct but small downgrowth on its 

 inner side (r') obviously representing the ventral continuation of 

 the dental lamina and the undeveloped permanent tooth of Kiiken- 

 thal, Eose, and Leche, and according to the interpretations of these 

 observers this small calcified tooth must be a vestigial milk-tooth. 

 The tooth itself consists of dentine and possesses an irregular 

 contour and a variable pulp-cavitj^ ; the relation of the odontoblasts 

 suggest that they are concerned more in absorption than in forma- 

 tion of tooth-substance. 



The dental lamina, after giving rise to these two teeth, disappears 

 for a few sections, but soon reappears and swells out again to form 

 the 2nd functional tooth, which 1 at first took to be the 3rd incisor. 

 However, on investigating carefully the relation of the extremely 

 reduced 2nd rudimentary tooth (fig. 3, i^), it was obvious that the 

 enamel-organ of the latter was connected with the gum slightly 

 antei'ior and external to that of the former, although, owing to the 

 large size and swollen nature of the enamel-organ of the 2nd 

 functional incisor, part of this tooth appears in sections anterior 

 to the rudimentary one ,: but the only important point to be ascer- 

 tained in determining the morphological relation of teeth is not 

 which tooth is anterior to the other, but which enamel-organ 

 arises in front of the other from the dental lamina. 



This 2nd rudimentary tooth is then the morphological 3rd 

 incisor, the 2nd functional one being the 4th. 



The 3rd incisor is, like the 2nd, a vestigial structure only present 

 in the embryo and although possessed of an enamel-organ it has 

 no enamel ; its body, consisting of dentine, is very ii'regular and is 

 obviously being reabsorbed. It is smaller and still more vestigial 

 even than the 2nd incisor and shows no ti-ace of any downgrowth 

 from its enamel-organ, but from its resemblance to the 2nd 

 incisor I conclude that it also belongs to the 1st dentition. 



The remaining incisors are very interesting and present such 

 striking relationships that they call for a very careful examination, 

 especially in the light of Eose's statement regarding the 3rd adult 

 incisor. 



Pkoc. Zool. Soc— 1893, Xo. XXXI. 31 



