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Show 502 MR. O. THOMAS ON THE GENUS ECHINOPS. [June 14, zygomatic processes) 167 ; interorbital breadth 9-1 ; palate, length 19-1, breadth outside p*_ 12*2, inside p4^ 4*7 ; length of lower jaw from condyle (bone only) 2 7 ; 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. Unfortunately 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 Echinops, 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 Centetes and Ericulus, and therefore the missing premolar of the full set of four may be presumed to be p\ 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 Echinops is therefore 0 2 3 4 { 2 3 4 M. rl 1 2 1 2 3 4 1 2 0 1 0 2 3 4 1 2 = 24.32\ 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 . l^\, the totals therefore being 24. 36. Two points about tbe 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 resemblance so great that it is extremely difficult to say whether anv 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 rather than to the permanent series2. 1 This ingenious method of so writing the dental formulae as to show clearly both the milk and permanent teeth and their relations to one another is copied from Dr. Winge's paper " O m 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 & 658 (1891), and (for Placentals) Thomas, Ann. Mag. N. H. (6) ix. p. 310 (1892). |