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Show 104 DR. C. I. FORSYTH MAJOR ON PLIOCENE VOLES. [Feb. 18, speak, of the fossil tooth. I had before met with a similar feature in one of two very young teeth of the recent amphibius-group, from Pisa, which presumably belonged to Savi's Arvicola destructor. In the recent tooth the enamel islet showed a slightly different position and genesis, was quite superficial and therefore ephemeral; it was associated with some other complications- two additional shallow enamel-loops-which likewise approach the tooth of the very young amphibius to that of the Pliocene form. In the recent species this pattern is very soon worn away ; there is no more trace of it in slightly older specimens. This is a fresh instance of a recent form preserving in the younger stages of its molar the features of a Tertiary form. W h e n during the revision of the fossil Rodentia of the British Museum, the Microtidse of the Forest Bed came to be studied, I was anxious to ascertain whether the enamel islet occurred there too in adult specimens, as is the case in the Val d'Arno fossil. Mr. E. T. Newton has published an elaborate description of the Rodents of the Forest Bed and Norwich Crag, and has shown that the larger Voles are, by the presence of well-developed fangs to their molars, very distinct from the amphibius-type with which they had been confused by all previous writers. He who enjoys the advantage of standing on his predecessor's solid shoulders, has also the duty to try and see a little farther, especially when additional material has accumulated in the meantime. If, therefore, to-day a step forward is possible in the knowledge of the Pliocene Voles, it is but fair to acknowledge that this is in a great measure due to Mr. Newton's previous work. Among the Voles' teeth of the Savin Collection from the Forest Bed, I found the character alluded to, but only in a relatively small number of teeth (e.g. text-fig. 13, no. 1) and in different proportions according to the localities. Whilst among 55 first molars from the West Runton Upper Freshwater Beds only four showed the character in question, the number of teeth provided with enamel islets was larger among the less numerous Voles' remains from the East Runton Forest Bed, and, moreover, other features became apparent. From the Norwich Castle Museum I have received of late, through the kindness of Mr. Leney, a small number of teeth and two jaws, here exhibited, which were collected by Mr. Fitch in the Norwich Crag at Thorpe (text-figs. 13, nos. 2-5; 14, no. 15; 15, nos. 20, 29). Here the presence of the enamel islet is the rule : there are ten anterior lower molars in this small series-eight exhibit the islet, one is very old and apparently has lost every trace of it; the tenth, a very young tooth (text-fig. 15, no. 29), reveals the genesis of the islet, which is the central portion of the antero-external enamel fold. Moreover, the teeth are of two different sizes. In short, the result of the investigation is, that the Voles of the Norwich Crag are different from those of West Runton and are represented by two species; whereas at East Runton the |
