502 MR. O. THOMAS ON THE GENUS ECHINOPS. [JuTie 14, 



zygbinatic jirocesses) 16"7; interorbital bteadth 9-1 ; palate, length 

 19-1, breadth outside p^ 1 2*2, inside p;^ 4-7 ; length of lower jaw 

 from condyle (bone only) 27 ; height from coronoid to angle 13'4. 



Of the five specimens examined, two others are of almost 

 precisely the same size as the type, and two, of similar age, are 

 rather smaller, the difference being, I presume, sexual. Unfortu- 

 nately the skins and skulls are not identified with each other, so 

 that I cannot say for certain whether in this form the male or the 

 female is the larger. 



Now with regard to the homologies of the teeth of JEchinops, the 

 five cheek-teeth of which have been supposed to be two premolars and 

 three molars, the reduction from the 3-3 of Ericulus being therefore 

 in the premolars, — I find on comparison that this is not the case, but 

 that the reduction is in the molars, of which there are only two, 

 while the premolars number three. These three premolars all have 

 milk predecessors, as is also the case in Oentetes and Ericulus, and 

 therefore the missing premolar of the full set of four may be pre- 

 sumed to be pS which is as yet not known to change in any member 

 of the Linnean group of Ferae. The incisors are two, above and 

 below, both in the permanent and milk series. 



The full dental formula of Ecliinops is therefore 



2 3 4 ("12 



' ' ' MA l = 24.32\ 



2 S 4 



2 3 4 [12 



Comparing this with the formulae of some of the allied genera, we 

 find that Ericulus is in all respects the same except that it has 

 M. ^, the totals therefore being 24. 36. 



Two points about the milk-change of Ericulus may be specially 

 noticed, both of them showing a very low and uuspecialized 

 condition. The first is the extraordinary resemblance existing 

 between the milk-teeth and their respective successors, a resem- 

 blance so great that it is extremely difficult to say whether any 

 given set of teeth belongs to the milk or permanent series. And 

 this difficulty is increased by the second point, namely the fact that 

 the molars come up with and stand perfectly in series with the milk 

 premolars, the last molar being fully up and in use some time before 

 these commence to fall. This fact, in so lowly an animal, is 

 decidedly confirmatory of the recent suggestion that the molars even 

 of the Placentals really belong to the milk father than to the 

 permanent series ^ 



^ This ingenious method of so writing the dental formulse as to show clearly 

 both the milk and permanent teeth and their relations to one another is copied 

 from Dr. Winge's paper " Om Pattedyrenes Tandskifte" (Vid. Medd. 1882, 

 p. 15). The method is so clear that no explanation is needed. 



2 Cf. (for Marsupials) Kiikenthal, Anat. Anz. vi. pp. 369 & 668 (1891), and 

 (for Placentals) Thomas, Ann. Mag. N. H. (6) ix. p. 310 (1892). 



