THE AQUATIC COLEOPTERA OF THE MID-EBUDES 77 



has apparently at some time been joined to it, as the channel 

 between the two is narrow and shallow, continues the line 

 to the south-west. 



Along the side exposed to the N.W. are several patches 

 of sand-dunes. The hollows between the hummocks of rock 

 are apparently filled with boulder clay, and in parts, especially 

 towards the south-western end where the ground has been 

 drained, there is excellent grazing land, s but the greater part 

 of the island is covered with shallow* peat in which are 

 innumerable small pools where Sphagnum and Eriophorum 

 are the dominant plants. 



On the island, therefore, the " peat-moss " or " oxylophil " 

 fauna is dominant, but some species of this group are 

 absent owing to the absence of high ground. Other species 

 of the group are absent or very rare owing to the majority 

 of the peat pools being very shallow. For instance, Ilybius 

 cenescens, Thorns., was only found in a few deep holes near 

 the road to the south end of Arinagour, holes which had 

 been dug to provide ballast for the road. Dytiscus punctulatus, 

 F., only occurred in one or two deep holes, as did also 

 Agabus chalconotus, Panz. The shallowness of the peat 

 accounts for the absence of deep holes at the peat-cuttings, 

 so that even the dominant group of the water-beetle fauna 

 is not fully represented. 



One or two ditches and pools in the sandy regions 



produced members of the typical freshwater-marsh fauna, 



such as Hydroporus unibrosus, Gyll. ; palustris, L. ; Colymbetes 



fuscus, L., etc., while one or two small streams on the same 



ground produced Hydroporns discretns, Fairm., 1 and Agabus 



1 I have several times been asked as to the characters by which I separate 

 H. discretus, Fairm., from H. nigrita, F., on the one hand, and H. pubescens, Gyll., 

 on the other. From H. nigrita it is at once separated by the sculpture of the 

 elytra. In H. nigrita these are punctured, but the whole surface is dull owing 

 to its being covered with fine reticulations. (It is described by Fowler and others 

 as being finely coriaceous. ) From the punctures arise fine short hairs, but these 

 are so inconspicuous that the insect appears to be glabrous. In H. discretus, the 

 elytra are punctured, the punctures being set perhaps a little closer together than 

 in H. nigrita, but the general surface is smooth and shining. From the punctures, 

 however, long hairs arise so that the insect is evidently pubescent. 



The form of H. discretus, which somewhat resembles that of H. nigrita, is at 

 least sufficient to make one look closely at a specimen before naming it H. pubesceiis, 

 but there are better characters for distinguishing it from this latter species. In 

 the first place in H. pubescens the prothorax is smooth between the punctures 

 except towards the anterior border where it is marked with very fine reticulations ; 



