722 



become visible and the teeth so evolved remain also in liieir develop- 

 ment a little in advance of those of the endoslichical row In Mam- 

 mals tiie same structural principle is met with. Here also the Anlage 

 of the dentition is in a bnccal — exostichos — and in a lingnal 

 row or endosticlios and the elements of the two rows alternate as 

 with the Reptiles. Hence the dentition of both groups of vertebrates 

 is distichical in Anlage. In the course of its furtiier development 

 however essential differences arise between the dentilions of Reptiles 

 and Monodelphian Manitnals. 



With the Reptiles the endostichical teeth are regularly intercalated 

 between the e.vostichical, .so that in the functionating dentition the two 

 rows are tixed on or in tiie jaw in a single row. So it is charac- 

 teristic of the morphology of the reptilian dentition that in it the two 

 rows of teeth fuiictionate simultaneouslj-. 1 should like to express 

 this fact by calling the reptilian dentition "hamastichical". And since 

 at any rate in the beginning between every two exosliehical teeth 

 an endostichical one is inserted and takes part in the construction 

 of the dentition, the mixing of the rows being thus a regular one, 

 also this fact might find expression in the characterisation of I he 

 reptilian dentition. Hence the functionating dentition of the Reptiles 

 should be described as an "isocrasic hamastichical" one. 



In regard to this characteristic a fundamental difference is now 

 met with between Reptiles and Monodelphian Mammals. Although 

 also with these latter the two dental rows evolve shortly after each 

 other, still the inner one or endostichos generally develops much more 

 slowly and its elements do not push themselves between those ot 

 the exostichos. This latter forms a compact dentition of which the 

 teeth pierce with a certain regularity and functionate during some 

 time, while the teetii of the endosticlios remain below the surface, 

 developing slowly. When they have reached a certain degree of 

 development they gradually expel the teeth of the endostichos, i. e. 

 the milk-set, till up their places and form the second or permanent 

 set. The typical dilference between the reptilian and mammalian 

 dentition consecpiently is that the functionating dentition of the former 

 consists of elements of both rows, while on the other hand with 

 the mammals the two rows functionate one after another. Hence I 

 distinguish the dentition of the Monodelphian mammals as c/wrLssfi- 

 cliical as compared with the hamastichical one of the Reptiles. It 

 should be pointed out however that according to the investigations 

 of Leche the functionating dentition of Erinacidae also consists of a 

 mixture of elements of the rows. Probably this expresses the very 

 primitive character of this animal group. 



