found the anterior part of a skull (Pl. VIII. fig. l,a,b,c,d) and several lower jaws of a larger form. These appear to be undistinguishable from those of Arvicola ratticeps=Lemmus medius' (Neilson), the recent animal being an inhabitant of Russia, Western Siberia, and Norway.
The lower jaw is easily recognizable by the formation of the anterior molar (Pl. VIII. fig. 1, d), in which the two anterior columns coalesce on the external surface, while they are well defined internally. The upper dentition closely resembles that of Arvicola agrestis; but the posterior molar differs slightly, but constantly, in form (P1. VIII. fig. 1, a). Excellent plans of the teeth of these four forms are to be found in Blasius, 'Wirbelthiere Deutschlands,' from which a clearer idea of the minute differences which characterize the species can be obtained than from the most detailed description.
5. Five lower jaws (Pl. VIII. fig. 2 a, b) exist in the Taunton Museum, which we cannot refer to any recent or fossil form with which we are acquainted: they are nearly as large as those of Arvicola amphibius; a depression usually found on the internal surface of the mandible, under the alveolar border of two anterior molars, does not exist in these jaws; this gives the jaw a peculiarly robust appearance. The chief peculiarity is in the dentition, which closely resemblesthat of Arvicola subterraneus (De Selys), one of the most minute forms of the genus. The anterior and characteristic molar (Pl. VIII. fig. 2 b) has six equally developed columns on the inside, and five externally; these give, on the plan of the tooth, nine well-marked triangles, the anterior and posterior of which form both internal and external columns or, rather, buttresses. We have met with no recent teeth so large as these, with this amount of complication. It is possible that Arvicola ambiguus of the Breches de Coude, described by M. Pomel, may be of the same species; but we have had no opportunity of comparing the specimens, and have seen no description of the teeth; we therefore propose to name the form provisionally Arvicola Gulielmi, after the energetic explorer of the Somerset caves, the Rev. D. Williams.
6. Genus Lemmus. — We find six lower jaws, which most closely resemble those of Lemmus norvegicus (Desmarest); they are, however, slightly smaller, and the condyle, with its neck, is slightly more slender in proportion to the size of the jaw: we cannot, with our present means of information, ascribe to these differences a greater than varietal value (Pl. VIII. fig. 3, a, b).
7. The anterior portion of a skull exhibits a form of dentition and some other peculiarities which show the animal to which it belonged to be as closely allied to Lemmus torquatus (Desmarest) as the before-mentioned jaws are to Lemmus norvegicus: the difference appears to be solely that of size, the fossil being larger than the recent specimens in the British Museum with which we have compared it. It is probably identical with the animal which Dr. Blackmore found in the Fisherton deposits near Salisbury. The posterior upper molar of Lemmus norvegicus and Lemmus obensis is of a totally different form from that of Lemmus torquatus and groenlandicus, the
