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On Ilyocryptus agilis (Kurz), a Rare Mud-Inhabiting 

 Water-Flea. 



By D. J. SCOURFIELD. 



(Read June 15th, 1894.) 



Plate XXII. 



In May last year, upon the occasion of the Club's visit to the 

 Royal Botanic Gardens, Regent's Park, I was very pleased to find 

 Ilyocryptus agilis (Kurz) in the well-known Victoria Regia tank. 

 This species had not been previously seen anywhere in this country, 

 and although its occurrence in the tank does not give it a claim to 

 be considered as British, yet since it has been recorded by at 

 least two Continental observers it is to be expected that sooner or 

 later it will be legitimately added to the British fauna. Under 

 these circumstances the following short description may help to 

 facilitate its recognition by collectors. 



In general outline Ilyocryptus agilis (Plate XXII., Fig. 1) does 

 not differ markedly from the normal type of the genus as exem- 

 plified, for instance, by our widely distributed and fairly common 

 /. sordidusy Lievin. The head is, however, as seen from the side, 

 rather smaller and perhaps a little more pointed than in the last- 

 named species, while the shell is a trifle broader, being at least as 

 broad as long, with its posterior margin almost straight. The dorsal 

 margin, moreover, is strongly keeled. Seen from the front the 

 animal appears more compressed than /. sordidus. Very notice- 

 able points in connection with the shell ^f this species are its com- 

 parative cleanness and transparency, both due to the fact that the 

 animal's successive moults are complete. This further accounts 

 for the absence of the concentric rows of spines on the shell 

 which are so characteristic of /. sordidus. The shell-sculpture 

 consists of a very faint network of rather large hexagons, formed by 

 lines of fine dots (Fig. 2) . In the specimens I had under examination 

 a number of small oil-globules, between the outer and inner layers of 

 tlie shell-valves near their ventral margins, formed a somewhat con- 



