224 Proceedings of the Royal Irish Academy. 



to them. The spongin is very scanty, so that the entire skeleton is weak and 

 is loosely held together. The whole appearance of the skeleton is thus a 

 great contrast to that presented by robust fonns of the species, in which both 

 main and transverse fibres may reach a diameter of 015 mm., and may 

 possess a considerable quantity of spongin closely binding the spicules. 



The skeleton -spicules are long, but slender. They usually vary between 

 0'22-0"33 mm. in length, and have a maximum diameter of 001 mm. In 

 some cases the maximum diameter is about U 006 mm. 



The free microscleres are also slender, being about 0'002-0004 mm. in 

 diameter. Usually they are few in number, but sometimes they are present 

 in fair quantities. 



Gemmules are present as a rule in considerable numbers, and may appear 

 as early in the year as June. They are usually without a grauular layer, or 

 with this layer very feebly developed, and are of a clear pale yellow colour. 

 They vary considerably in size, but do not appear, on the whole, to be smaller 

 than in more typical si)ecimens. 



The gemmule-spicules seem to be absent from some specimens ; in others 

 they are present in scanty numbers. They measure, as a rule, between 

 0-008-0 l.S mm. by 0003-000o mm. 



In lakes in which this small form grows there may sometimes be found 

 small green tinger-like specimens, perhaps only half an inch in height, on the 

 sides of the stones. In one lake only, namely, in Lake Nacorra, County 

 Mayo, were long branching green s{)ecimen8 found, about a foot in height. 

 In these sponges the bi-anchcs were ver}" soft and slender. The spicules, too, 

 were slender, quite resembling those of the small encrusting specimens 

 growing under the stones. 



The small encrusting fonn of ^. lacustris is described at some length by 

 Dr. Annandale (2) from specimens found by him on the under surface of 

 stones in Loch Haa, in the Isle of Mull, Scotland. It is considered by him 

 to be possibly a distinct local race. 



This phase of H. lacuMrU, so characteristic in its extreme form, is connected 

 by every intermediate link with other specimens of S. lacuMris which grow as 

 more robust encrustations on the under surface of stones in lakes and streams 

 on both calcareous and non-calcareous rocks. It has been traced from the 

 lakes in which it grows down the streams that drain the lakes. As is the 

 case with other species, iS. /atin/nis grows most loxuriautly in these streams at 

 a point a little distance below the lakes. Hence in such situations it extends 

 in much larger patches on the under surface of stones ; but there is no 

 appreciable difTereuce in the size or shape of the spicules, or in the relative 

 abundance of the gemmule-spicules. 



