1896.] MAMMALIAN DENTITION. 565 



seem rather strange, but we must bear in mind the fact that this 

 cusp is apparently of little importance in Erinaceus, as it is very 

 small in adult and may be almost wanting on m72. 



A further consideration of these cusps will be found at the end 

 of this paper. 



The relations of the milk and permanent teeth of Erinaceus may 

 be represented as under, the reduced teeth being indicated in 

 italics, those which never cut the gum and are entirely functionless 

 being enclosed in bi'ackets, while the functional ones are repre- 

 sented by ordinary figures (Winge, 26) : — 



ri23 ri ro23 4i2 4 fi 2 s 



iJ LJ_W. oJ i-- pJ 5- 



■^•^1 (i) 2 {3) ' ^-^ (1) ' -^ -^ (3) (S) 4' "^ (i) (3) 4 ' 



2 (S). nil. p J (g) 3 id) 2 (3) 4 . , , J 



u 



1^23 11 1.0 2 41 4 U23 



Gtmnuea. 



So far as I am aware, no young specimens of this genus have been 

 examined in the flesh for their tooth change. Thomas (23) has, 

 however, published a bare statement of two dentitions in this genus, 

 based, I believe, on a young, dried skull in the British Museum 

 collection ; but there is a good deal of uncertainty attached to this 

 method, for although the jaw has been cut to expose the underlying 

 tooth-germs, no actual germs are visible, and one can only surmise 

 their existence from the presence of cavities at the roots of the 

 functional teeth (Plate XXIV. fig. 12), and by a comparison of 

 these teeth with those of an adult specimen. 



The dentition, according to Thomas, is : — 



ri 2 3 ri fl 2 3 4 f 



1 2 3 . p,J 1. pj g 3 4 . ,f J 

 W 2 3' ^-i'l' ^-^ 3 4' *M 



fl 2 3 



i-^jW-i' c-|i; P.<j 



1.^1 2 3 I, 1 t,l 2 3 4 U 2 3 



<li-3 and dp'"-2 being vestigial. 



The specimen examined by me was a foetus, with a head length 

 of about 49 mm. and a total length of 205 mm. 



The Incisors. 



I- 1 & i.2 are large and well calcified, each showing a marked 

 lingual development of the dental lamina, indicative of a suc- 

 cessional tooth. On the other hand, i^3 is more specialized, and 

 only present in the permanent set of teeth. In my foetus this 

 tooth was very backward in its development, its enamel-organ being 

 but slightly differentiated (pi-^, and exhibited on its labial side a 

 large irregular calcification (fig. 10, di. 3 ), provided with a reduced 

 enamel-organ; this is evidently the milk predecessor of i^, and 



