220 Proceedings of the Royal Irish Academy. 



rule, less luxuriantly in lakes on the limestone still holds good, but it must 

 be noticed that they sometimes grow in abundance on the limestone in rivers 

 which drain a lake as just described. In the localities quoted, both the 

 River Shannon and the Drumcliff Eiver flow over the limestone at the spots 

 indicated. But it must be remembered that the Eiver Shannon at the place 

 described drains Lough Allen, a large lake lying for the most part on the 

 Lower Coal Measures, and that the bed of the river was largely made up of 

 slaty fragments carried down from the Coal Measures. Glencar Lough, 

 however, lies on the limestone. 



The stones in many limestone lakes are covered with a thick, soft, 

 calcareous deposit which seems to afford an unfavourable substratum tor the 

 growth of sponges, and which may be the cause of their scarcity in those lakes 

 (41, p. 4). Spongilla laaistris and, more rarely, Ephydatia /luviatilis are able 

 to establish themselves on such calcareous deposits, but they do not flourish 

 on them. 



As regards the second point, Ileteiomei/cnia Rijdtri has not Ihhmi fnund 

 growing on the limestone in any part of Irelaml. It occurs on all sorts of 

 non-calcareous i-ocks — granite, sandstone, mica-schist, basalt, and felstone. 

 As suggest^'d in a former pajier (41, p. 4), llie reason fur the absence of 

 HeUromeyenin Ri/deri from liinestone areas may, perliaps, be a pliysieal one. 

 The favourite habitat of the species, for the most part the only one in Ireland, 

 is the under .surface of stones. In lakes on non-ealcareous rocks the stones 

 arc clean, and lie loosely on one another, thus allordingslielter from the light 

 and a free access of water to the sponge. In tlie lakes on the limestone the 

 stones are often half-buried in mud, and in addition are often covered with a 

 tliiek liinv iloposit. Sucli conditions would seem to be totally unfavouiable 

 to the growth of llrtrnnnr.ijenin Ilydcri. On llie otlicr hand, tlic species is 

 equally absent from lakes and rivcre where the limestone is cleaner, and 

 affords freer under surfaces to the growtli of sponges. Heteromci/cnia 

 lii/dtri has not l)een found in tlie lakes which lie partly on the lime.stwno and 

 partly on non-calcareous rocks. The only CNception to this statement, up 

 to the present, is the finding of two small specimens of the species in the 

 extreme north-western ann of Lough Corrib, which large lake lies for 

 the iniMt |>art on the limestone. But this north-western arm lies on 

 non-calcai-eous rocks, and i^eceives the drainage of the surrounding 

 Don -calcareous country ; it is united only by a narrow channel with tlie main 

 body of the lake, so that this part of Lough Corrib is to all intents a separate 

 lake. The species wa.s lookeil for in vain in the main Ixuly of the lake, which 

 lies on the limesUiue. 



As a general rule, Spow/illa lacuMris and Helcroiiuijenia Uijderi grow side 



