170 



VI. — On Pyxidicula invisitata, a Ehizopod new to Britain, and 

 Hedriocystis spiniftra, a neiv Heliozoon. 



By James Meikle Brown, B.Sc, F.L.S., F.C.S. 



[Communicated by John Hopkinson, F.L.S., F.R.M.S.] 

 {Read April 17, 1918.) 

 One Plate. 



During the examination of material collected in different parts of 

 the country for the purpose of studying the distribution of fresh- 

 water Khizopods, 1 have frequently come across small, inconspicuous 

 species whose identity is difficult to determine, partly owing to 

 their diminutive size and partly to their almost transparent nature. 

 For these reasons it is also difficult to obtain a clear idea of their 

 structure. Naturally these minute creatures are frequently over- 

 looked. 



Amongst such there commonly occurs a species of Pyxidicula 

 which has not yet been reported as occurring in this country. It 

 seems to be referable to the species P. invisitata Averintzeff, 

 described very briefly in 1906 (" Arch. f. Protistenk,," Bd. viii. pp. 

 86-7). 



Since the publication of that paper I have seen no further 

 reference to this species. As it appears to be widely distributed in 

 Britain, a fuller description of it should be of interest. 



EXPLANATION OF PLATE. 

 Figs. 1-4, — Pyxidicula invisitata Averintzeff. 



FIGS. 



1. — Dorsal view of living animal. 

 2. — Transverse section of empty test. 

 3, 4. — Different forms of the marginal flanges, p.v., pulsating vacuole ; fl., flange 

 margin of test. 



All X 1000. 



Figs. 5-7. — Hedriocystis spinifera Brown. 



5. — Active individual, x 2400. 



6. — Portion of capsule, with spines. X 5000. 



7. — Portion of capsule and plasma, with nucleus and contractile vacuole, x 5000. 



