134 BRITISH FRESHWATER RH1ZOPODA. 



not as yet found in Britain do they appear to be set 

 in longitudinal series. The form of the test in this 

 variety may account for the difference, for it is quite 

 conceivable that a test approaching the spherical shape 

 would more readily lend itself to a longitudinal arrange- 

 ment of the surface-plates than one which is compressed. 

 The variety referred to var. globulosa was dis- 

 covered by Penard in the deep waters of the Lake of 

 Geneva and in the Lakes of Lucerne and Constance. 



In Q. irregularis a contractile vacuole may nearly 

 always be detected, collapsing and dilating at intervals 

 of about 45 seconds. The nucleus is small and not 

 very distinct. The protoplasm is colourless. A few 

 minute yellowish-green globules are always perceptible ; 

 but the whole organism is highly transparent, which 

 circumstance, together with its small size and slowness 

 of movement, renders it liable to be overlooked. As 

 a rule, in the Cheshire and also in the Isle of Man 

 examples the irregularity of the surface-plates, which 

 suggested the specific name, does not often occur. The 

 plates are as regular, certainly, as those of Q. syminetrica, 

 on the whole probably more so. The species appears 

 to be abundant where it occurs, but its distribution is 

 more sporadic than that of the related species. 



Penard has very carefully studied the forms of this 

 species which have been described by himself and 

 others under the varied names of Q. monensis, Q. glo- 

 bosa, Q. globulosa, and Q. discoides, and has come to the 

 conclusion in which we fully concur that they should 

 be included, as mere forms, under the common name of 

 Q. irregularis Archer. They seem to differ from one 

 another mainly in the disposition and size of the plates, 

 their setting, and the degree of rotundity of the test. 

 Penard also notes an interesting peculiarity in relation 

 to the chemical composition of the test. Lagerheim, 

 speaking of Q. globulosa, had (1901) pointed out that 

 this was distinguished from all other freshwater rhizo- 

 pods by the fact of the plates entering into the struc- 

 ture of the test being composed not of siliceous or 



