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Show 1874.] ON FOSSIL ARVICOLIDA. 465 brick-earth of the Drift period at Fisherton, near Salisbury, associated with Myodes torquatus, Spermophilus erythrogenoides, &c.; and these we find to be certainly the same as the Somersetshire specimens. A skull from the Bromberg cavern (from the Soemmering collection) is in the British Museum. On comparing the dental pattern of these fossils with Blasius's figures of A. ratticeps (Siiugeth. Deutschl. p. 366) we were struck with the great variety of form in the anterior part of the first lower molar, and more especially in the posterior part of the third upper molar. W e were at one time inclined to believe that the fossil might be regarded as a distinct species, and are still of opinion that it may represent a race distinct from the recent A. ratticeps; but the variations are so great, both in the recent and the fossil skulls, that we have sought in vain for any constant characters for specific distinction. In the first lower molar many specimens agree exactly with A. ratticeps, but in others the anterior extremity is produced beyond the first inner angle, so as to give the tooth the appearance of having eight cemental spaces instead of seven: this type is somewhat exaggerated in Mr. Sanford's fig. 1 d. The outer margin of the first two blended spaces is often less regularly convex than is usually the case in recent A. ratticeps, so that the whole tooth rather resembles Middendorff's illustration of A. obscurus (' Sib. Reise,' ii. pi. xi. fig. 4), and Hensel's of his A. ambiguus ( = A. brecciensis, see p. 466). But the most remarkable variation occurs in the third upper molar, which differs so much that it is only the numerous intermediate forms that convince us that all belong to the same species. A few, like that figured by Mr. Sanford, agree with typical A. ratticeps in having four external and four internal angles; but in many there are only three external angles, and the whole form of the tooth more resembles that of A. arvalis, while others present a type peculiar to themselves. All we have yet examined differ from A. brecciensis in having more than three internal angles (fig. 2, a-f). Fig. 2. Teeth of A. ratticeps and A. nivalis. |