[Illustration: _Pl. 71._ _H 109._ *Bank Vole.* Evotomys glareolus.] The form that Barrett-Hamilton recognises as _M. agrestis_ and calls the Northern Grass Mouse, is, so far as Britain is concerned, represented only in Scotland and its western islands by five sub-species which he names as under:-- Macgillivray's Grass Mouse (_M. agrestis macgillivraii_), a rich buff coloured form with thin fur, restricted to Islay, where it is rare. Hebridean Grass Mouse (_M. agrestis exsul_), common on several islands of the Hebrides. Distinguished from the Field Vole by its much larger size and duller brown colour. Eigg Grass Mouse (_M. agrestis mial_) restricted to the island of Eigg. Differs from _M. a. exsul_ in its shaggy coat of abundant long hairs. Highland Grass Mouse (_M. agrestis neglectus_) found on the summits of the highest Scottish mountains. It is larger than the Field Vole, with thicker fur and darker, browner upper side. It differs from _M. a. exsul_ in the simpler character of the first molar tooth. Muck Grass Mouse (_M. agrestis luch_), of which only three specimens have been taken, all on the island of Muck. About the same size as the Field Vole, it has a buff underside. *Orkney Vole* (_Microtus orcadensis_, Millais). So far back as 1805 the Rev. George Barry, in his "History of the Orkney Islands," mentions a rodent that was known locally as the Vole Mouse, which he believed to be the same as the _agrestis_ of Linnæus. He says it "is very often found in marshy grounds that are covered with moss and short heath, in which it makes roads or tracks of about three inches in breadth, and sometimes miles in length, much worn by continual treading, and warped into a thousand different directions." Towards the end of last century Mr. J. G. Millais obtained specimens, and on a critical examination found that they differed from the known forms in several details of skull structure and in the folds and angles of the teeth, sufficient in his opinion to constitute a new species, which he called _Microtus orcadensis_. It is larger than the Field Vole, with a longer and slightly broader head. It was found subsequently that specimens from different islands in the Orkney group showed differences due to their segregation over a long period, and they have consequently been distinguished as five sub-species. These differences are minute, and it would be wearisome and out of place in a popular work such as the present to detail them. Generally speaking, they are much alike, and their habits are practically identical, so far as at present known.