SANFORD——SOMERSET-CAVE RODENTIA. 125 
found the anterior part of a skull (PI. VIII. fig. 1, 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 an- 
terior molar (Pl. VIII. fig. 1, d), in which the two anterior columns 
coalesce on the external surface, while they are well defined inter- 
nally. The upper dentition closely resembles that of <Arvicola 
agrestis; but the posterior molar differs shghtly, but constantly, in 
form (Pl. 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. 24,5) 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 Arv- 
cola 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 appear- 
ance. The chief peculiarity is in the dentition, which closely resem- 
bles that of Arvicola subterraneus (De Selys), one of the most minute 
forms of the genus. The anterior and characteristic molar (Pl. VIII. 
fig. 2b) 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 pos- 
sible that Arvicola ambiguus of the Bréches de Coude, described by 
M. Pomel, may be of the same species; but we have had no oppor- 
tunity of comparing the specimens, and have seen no description of 
the teeth ; we therefore propose to name the form provisionally Arvi= 
cola G'ulielmi, after the energetic explorer of the Somerset caves, 
the Rey. 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, 6). 
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 com- 
pared it. It is probably identical with the animal which Dr. Black- 
more found in the Fisherton deposits near Salisbury. The posterior 
upper molar of Zemmus norvegicus and Lemmus obensis is of a totally 
different form from that of Lemmus torquatus and granlandicus, the 
