ORDINAL TYPES OF MOLARS: INSECT1VORA 117 



[NSECTIVORA. 



The first statements to be made are : (1) while Insectivora show 

 a persistent and prevailing tritubercnly or triangular arrangement of the 

 upper and lower cusps, it is also true that (2) the mode or sequence 

 of origin of these upper cusps, and the homologies of different cusps 

 and of different portions of the upper molar crowns are uncertain. 

 According to the evidence presented by Mivart (Proc. Zool. Soc., 1868) 

 and Gidley (quoted below pp. 12:5-126) in the trigonal molars of 

 Centetes, ChrysoMoris and other Zalambdodonta, the main cusp is 

 homologous with the paracone or paracone + metacone of other animals, 

 the protocone being represented by the internal ledge marked hij in 

 our Figs. 68, 69, B, c. 



Among extinct and modern Insectivora, owing apparently to the 

 persistence of very primitive feeding habits, we observe also the 

 persistence of very primitive stages of tritubercular evolution in 

 the molar teeth. The lower molars of certain Eocene and Oligocene 



FIG. 63. Skull and dentition of the Hedgehog (Erinaceus europcvus), a typical dilambdodout 



insectivore. 



Insectivores, such as Apternodus and Micropternodus, are practically 

 identical in pattern with those of the Jurassic Amphitlierium, Amblo- 

 therium, and Dryolestes, presenting an extraordinary instance of persistence 

 of type. 



Insectivores retaining these primitive tritubercular molars have 

 been distinguished by Gill as Zalambdodonta (with a single external 

 lambda or crescent), whereas the more specialized Insectivores with 

 quadrate or subquadrate molars, such as Erinaceus, have been termed 

 Dilambdodonta (with two external lambdas or crescents). There is 

 no doubt that the quadrate or dilambdodont condition is secondary 

 because we have, first, analogy with other groups in which the 

 quadrate is always found to succeed the tritubercular form ; second, 

 there is the direct evidence in the unworn molars of JSrinaceus, and 

 especially of G-ymnura, and some of their fossil predecessors, as shown 



