230 Proceedings of the Royal Irish Academy. 



not of a vigorous growth. It was in the form of small, thin patches, growing 

 chiefly on the under surface of stones. Numerous gemmiiles were present, 

 which were furnished with slender, quite normal amphidiscs. The absence of 

 bubble-cells was another character which prevented any confusion of these 

 specimens with E. Mfilkri. 



In specimens of E.fluviatUis that may be regarded as typical the megas- 

 cleres do not vary very much in size and shape. In such examples they are 

 fairly stout, slightly curved, smooth o.xea, tapering gradually to a sharp point 

 at each end. Some have a very slight swelling in the centre of the shaft, 

 and a very few are microspined (1*1. XXVII, fig. 1). It may be mentioned 

 that Topsent (48) lias already noted that there is often a slight swelling in the 

 centre of the oxea of typical specimens found in France. In other specimens 

 the oxea are similar to the foregoing, but are much more slender. 

 Tiiey may be gradually pointed, as in specimens fiom Lough Beltra, 

 Co. Mayo, or abruptly pointed, as in examples from Furnace Lough, in the 

 same county (I'l. XXV II, (ig. 2). .Some of the oxea have a central swelling, 

 and a numt>er are very minutely spiued. In specimens from the pond in the 

 Zoological Gardens, Dublin, as already mentioned, the rather slender, spined 

 spicules arc very numerous (PI. XXVII, fig. 3). 



Again, other specimens of H.^tlnrintiUs have typical mcgaselercs, namely, 

 fairly liiick, slightly curved, gradually pointed, smooth oxea; but among 

 these is a considerable admixture of straight, or nearly straight, spicules 

 which are shorter and thicker, and which taper abruptly to a point at either 

 end. Some of the spicules liavc a slight central swellijig, and a few are 

 microspined (ri.XXVlI, lig. 6). These lead on to other specimens, in which 

 the majority of the ni^asclercs are short, very thick, nearly straight or 

 slightly curved spicules, which taper abruptly, or more rarely gradually, at 

 either end (PI. XXVII, fig. 4). A few of these are microspined. When 

 short, thick spicules, either microspined or smooth, are formed to the complete 

 or almost complete exclusion of the longer, more typical oxea, a peculiar 

 form of Ephydatia ftuvicUUxa results, which, for the sake of clearness, will 

 be dealt with later on. 



As well as the variations just described, abnormally formed spicules may 

 be present in greater or less numbers in any specimen of E. fluvialilis. For 

 example, the oxea may have one end rounded off, or even knobbed, or a 

 series of swellings may be present along the shaft of the spicules (usually 

 in slender, poorly developed spicules, and in young spicules), or the oxea 

 may be reduced to a sphere, with or without one or two spike-like projections. 

 Weltner (53) gives figures of some of these abnormalities, 



Willi regard to the measurements of llie foregoing spicules, the more 



