628 



Neither do I think there is the slightest evidence to bear out his 

 statement that he has seen traces of an additional incisor behind the 

 2nd upper one of the Rabbit. He is evidently in complete ignorance 

 of the works already cited with regard to the development of these 

 teeth or he would not assert as he does that the incisors of the 

 Rabbit and Hare undergo no change. 



The wild statements which he makes with regard to the supposed 

 milk dentition of numerous genera of simplicidentate Rodents are 

 hardly worth notice save that they might otherwise be supposed to 

 be founded on a more solid basis than the presence of a few minute 

 pits in the jaws (probably vascular in function) which are very vari- 

 able both in size and position ; from these structures however he ad- 

 duces a most elaborate set of dental formulae and provides these 

 animals in their early life with perfect ses of (false) teeth consisting 

 of incisors, canines and molars. 



A little more trust worthy evidence as to the existence of ad- 

 ditional teeth is needed before we can accept these dental formulae 

 as having any scientific value. 



The molars. 



I have carefully examined sections of the developing molars of 

 the Rabbit, Guinea Pig, Rat and Mouse 16 ), with a view to ascertain 

 whether these teeth were to be refered to the first (milk) dentition 

 as affirmed to be the case for the molars of the polyprotodont by 

 Leche and Rose * 7 ) , or to the second dentition as I have been 

 led to conclude was the condition of the molar of the Macropodidae. 



In the Rabbit I find that there is present on the lingual side of 

 the enamel organs of the 1st upper and lower molars a well marked 

 distal continuations of the dental lamina. That a similar structure 

 is present in connection with the 1st and 2nd molars of the 

 Guinea Pig (Fig. 3 e), while in the Rat and Mouse a very slight in- 

 dication of this structure is present in connection with the 1st cheek 

 tooth 18 ). 



16) Lepus cuniculuB (domestious), Cavia porcellus, Mus rattus (tame 

 variety) and M. musculus. 



17) I here purposely leave out of consideration Kükenthal's later 

 views (Biol. Centralbl., XII. Bd., pp. 400—413) that the molar teeth of 

 all Mammals represent a fusion of at least two distinct dentitions, for the 

 reason that I have not so far found any evidence to justifiy the theory. 



1 8) When only one molar is developed and it exhibits a well marked 

 lingual downgrowth of the dental lamina, great care must be exercised 

 before describing this structure as representing an indication of a sue- 



