240 Proceedings of the Royal Irish Academy. 



• Gkoup I. 

 Heteromei/enia Ryderi Potte. T3pical or Eivei Form. 



This foiTU occurs in rivers and streams, and corresponds to the f,}pica 

 Hderomcyaiia Bydcri, described from specimens taken in " shallow, flowing 

 water " in North America. 



In this country the sponge grows nearly always under the shelter oC 

 stones, but sometimes spreads from them to envelop the stems of water- 

 plants. Occasionally it is found on dead, submerged branches. The smaller 

 specimens are more or less circular in outline; they are thickest in the 

 middle, and thin out towards the edges, so that the upper surface is some- 

 what dome-shaped. The larger specimens spread out into lobed, encrusting 

 masses of irregular shape, but sometimes of considerable size. The sponge is 

 pale yellowish in colour, veiy soft to the touch, and very fragile. The 

 surface is even ; but under the lens the dermis is seen to be raised up on the 

 tips of the main skeleton-fibres into very minute points. In preserved 

 specimens, at least, the main fibres sometimes pierce the dermis and project 

 very slightly. The oscula are small and inconspicuous, being about 1-2 mm. 

 in diameter. 



The skeleton is made up of main fibres, usually about 002-003 mm. in 

 thickness, which run upwards through the sponge and occasionally Inauch. 

 They consist of spicules in usually three to four rows. Wlien the spicules 

 are more slender, a greater numlicr of them lie side by side. These main 

 lihres are the length of one spicule ajwirt, and are united by spicules at right 

 an^'les to them, which usually lie singly, but sometimes are in bundles of two 

 or three. These transverse fibres do not fonn continuous fibres. In places 

 the skeleti'U is confu.seil, but usually becomes more regular towards the 

 surface of the sponge. 



Spongin is very scanty in quantity. 



The skeleton-spicules are slightly curved, occasionally straight oxea ; 

 tliey taper evenly to Iwth ends, which are pointed. The shaft is thickly 

 covereil with rather s^niall spines, except at the extreme tips, which are 

 smooth. Unlike the skeleton-spicules of the lake form of the species, these 

 oxea are very constant in shape, and vary only in length and thickness from 

 one specimen to another, lliey measure usually from 2-0'33 mm. in length, 

 with a maximum diameter of OOl mm., or even 0013mm. 



The longer gemniule-spicules have a straight shaft, usually rather thickly 

 set with strong spines, which are straight or curved. At either end of the 

 spicule are four to six terminal, strongly cur\'ed spines. These spicules 

 ni«'a.siin^ OOi-OOfio mm. in length. 



