Stephens — The Fresh-water Sponyes of Ireland. 239 



Ireland the sponge grows in situations sheltered from the light, nearly 

 always under stones. It is pale yellowish or greyish-white in colour. On 

 the rare occasions on which the sponge was found in places where a certain 

 amount of liglit penetrated to it, thei'e was still no trace of any green coloura- 

 tion. One or two specimens indeed were taken which were dull green in 

 colour, but these were penetrated in every direction by a green filamentous 

 alga. 



H. Rydcri is only found in Ireland on non-calcareous rocks. The North 

 American localities for the species are not given in sufficient detail to enable 

 one to decide if it always avoids limestone areas in that continent. 

 Dr. MacKay, the discoverer of the pictovensis form of the species, writing 

 from Nova Scotia, informs me that so far this form appears to be found in 

 non-calcareous regions in that province. Potts states that the species has 

 been taken chiefly in the States, along the Atlantic coast. The eastern 

 maritime States of North America are for the most part free from limestone, 

 so that it is possible tliat the species avoids calcareous rocks in North 

 America as it does in Ireland. 



As H. Rydcri avoids the limestone, its distribution in Ireland is very 

 striking. It is absent from the whole centre of the country which consti- 

 tutes the Great Limestone Plain of Ireland, and it is confined to those parts 

 of the maritime counties which are formed of non-calcareous rocks. It is 

 not confined to the west, as was thought on its first discovery in Ireland, but 

 occurs in the north and south, as well as in the east. It grows in low-lying 

 lakes and rivers, as well as in mountain tarns and streams. It is usually 

 the only species found in the higher mountain lakes. The highest altitude 

 at which the species has been taken is 1,868 feet. 



As already stated, the various forms assumed by H. Hi/dcri in Ireland 

 approximate closely to three varieties of the species described by Potts from 

 North American specimens. These varieties are united by specimens 

 showing every possible gradation between them, yet the great majority of 

 the specimens obtained may be assigned to one or other of the three main 

 types. The arrangement proposed in the report on the fresh-water sponges 

 of the Clare Island Survey is therefore adhered to here for convenience of 

 description and of reference. Tlie three main types under which the 

 specimens are grouped are as follows : — 



Group I. — Hdcromeyenia Ihjdcri I'otts. Typical or Piiver Form. 

 Group II. — Hcteromcycnia Myderi Potts, toxin pidavensis, Voiis or Lake 



Form. 

 Group III. — Heteromcyenia Rydcri Potts, form Baleni, Potts. 



[2 Z)2] 



