338 Proceedings of the Royal Irish Academy. 



specimens of that species taken in a lake on Inishbofin, off the Galway coast, 

 which are here grouped with the Baleni form of H. liyderi. 



Some years before the Sable Island sponge was described, the discovery 

 of H. Rijderi in a lake in the west of Ireland was announced (20, 21). During 

 the following ten years the species was recorded from three or four other 

 localities in Ireland, and, finally, the recent field-work carried out in many 

 parts of the country has proved that it is widely distributed in Ireland in 

 nun-limestone areas, in which areas it is the commonest species of fresh-water 

 sponge. 



U. Eydcri is now known to occur in Scotland, where it was recorded by 

 Dr. Annandale under the name Tuhdla pcnnsylranica (see p. 215). 



Hetcromcycnia liyderi is well known to be a very variable species, autl its 

 extreme forms differ veiy much from each other both in external appearance 

 and texture, ami in the shape and size of the skeleton-spicules. That they 

 differ so much is shown by the fact that they received names as distinct 

 sjiecies : //. Ryderi Potts, H. picloveyisis Potts, H. Macouni MacKay. Potts 

 soon recognized the great variability of the species in North America, and 

 described liow, " in spite of an exceedingly rebellious disinclination," he was 

 forced to the conclusion that the forms which had passed through bis hands 

 must be regarded as belonging to one and the same species. 



When systematic collecting of fresh-water sponges was undertaken in 

 Ireland, it was i-ealized before long that //. liyderi was equally variable on 

 tliia side of the Atlantic; aud it is interesting to notice that tlie species 

 assumes closely similar forms in both countries (41, p. 9). We have the 

 typical //. Jiydcn from streams and rivers, the hard, compact form (var. 

 pidmriisU) from lakes, and the slender-spiculed form (var. lialcni) from lakes 

 in whirh the conditions are unfavourable to rol)ust growth of the sponge. 

 Tlio form with slender branches (var. U'fUsht) lias not yet been found in 

 Ireland. 



The spicules in both American and Iri.sh specimens are the same, except 

 that the macroscleres are slightly thicker, on the whole, in tiie former, and 

 the discs of the shorter gemmulc-spiculcs are lees deeply indented. Probably 

 the growth of the .sponge is more vigorous in every way in North America 

 than ill Ireland. Tlie slender-spiculed fci)Ocimen.s in both countries have the 

 shorter gemmule-spicules posses.«ing deejily indented discs. 



Although the forms are thus closely paralleleii in these widely separated 

 countries, yet there is an interesting difference in their mode of growlli. In 

 North America the various forms of tlic sfiecics grow in situations exposed 

 to the light (the first-found specimens were growing on the upper surface of 

 stones), and their colour is descrilied as liglit green or vivid green. In 



