29 14 Proceedings of the Royal Irish Academy. 



in the list — while one species with a southern and western distribution might 

 possibly be described as belonging to Watson's " Atlantic " type. 



A reference to the Continental distribution of these thirteen species shows 

 that seven of them have a range from Scandinavia to south Europe ; i.e., 

 D. griseo-striaius, A. congener, D. lappordcus, R. melanarius, H. dorsalis, 

 G. agilis and P. nigro-aeneus. The first three however owe their southern 

 distribution to the fact that they there inhabit highland lakes while the 

 fourth has, I think, two forms — a smaller highland and a larger lowland one 

 — which may account for its very wide range. H. dorsalis is a distinctly 

 northern species and like I), griseo-striatus and D. lapponicus has a holarctic 

 range, while the other two species, C. agilis and P. nigro-aeneus, are much 

 more common in the south than in the north. Two other species of the 

 thirteen are northern, i.e., 0. iic-lineatus and A. unguicidaris, and do not extend 

 south of Belgium and Xorth Germany, while the remaining four species are 

 distinctly southern not being recorded from farther north than about the 

 latitude of Denmark. 



The Continental distribution of these species therefore corresponds with 

 the Irish distribution while that in Britain differs from both only in the cases 

 of H. dorsalis and P. nigro-aeneus. Apart from these two cases we have 

 therefore in the western Irish group two clearly-marked elements and if we 

 analyse the distribution of the other seventy-nine "vvest Mayo species we find 

 that in them also we have northern and southern elements, and in some cases 

 anomalies with regard to British distribution. For instance, Caelambws 

 v-lineatus is a good example of a distinctly northern species abundant in 

 pools in Lapland, according to Zetterstedt (Insecta Lapponica, 1840), and not 

 found south of Denmark and north Germany. It is also a good example of 

 anomalous British distribution since it has occurred sparingly in various 

 parts of England, even as far south as the New Forest (Hants S.), while in 

 Scotland it has so far been recorded from only four counties — Kirkcudbright, 

 Fife, South Perth and Forfar. Of the markedly southern species, H. lituratus, 

 0. punctatus and probably 0. lejolisii may be mentioned, while a number of 

 others are also southern, e.g. H. lineatocollis, A r . sparsus, L. obscurus, H. ovatus, 

 D. elegans and xii-pustulatus, H. lepidv.s, puMscens and planus, A. nebulosus, 

 &c, &c. 



The western group is therefore chiefly peculiar on account of its " wester- 

 ness " but it also contains the two most arctic of all our British water-beetles — 

 D. griseo-striatus and D. lapponicus. The southern species are more southern 

 than the majority of the similar group of the generally distributed fauna so 

 that we may to some extent associate the western localisation with extreme 

 northern or extreme southern Continental range. This is significant from the 



