COSMARIUM. 173 



Diracleet and a Mhorgliain, Harris, Outer Hebrides ! 

 Plankton of Loch Beosetter, Bressay, Shetlands ! 



IRELAND. Small lakes between Clifden and Round- 

 stone, Galway ! 



Geoc/r. Distribution.- -QeTm&ny. Sweden. Finland. 

 N". Russia. Faeroes. Iceland. Australia. 



We do not think that C. ellipsoideum Elfv. differs sufficiently 

 from C. contraction Kirchn. to warrant its separation as a 

 distinct species, and we have therefore placed it as a variety 

 of that species. It differs from typical C. contractum in 

 having relatively shorter cells, with a less widely-open sinus, 

 and more exactly elliptical semicells. It is very variable in 

 size, and there are three forms worthy of special mention : 



(1) A form about half the normal size ; length 22 23*5 ju; 

 breadth 18-20 ju ; breadth of isthmus 6'5-7'6/u; thickness 

 11-12 /u. (PL LXI, figs. 30-32.) Not uncommon in the 

 Scottish plankton. This form is probably identical with 

 C. minutum Delp. ('Desm. subalp/ 1877, p. 105 (sep.), t. 7, 

 f . 37-39) , and perhaps would be better placed as C. contractum 

 var. minutum (Delp.). 



(2) A form of very variable size, with the sinus more closed 

 (= C. ellipsoideum Elfv. var. minor Racib. Nonn. Desm. Polon. 

 1885, p. 84, t. 10, f. 9; C. proteiforme Turn. Freshw. Alg. 

 E. India, 1893, p. 64, t. 9, f. 26); length 16-42 /u; breadth 

 11-29 ju ; breadth of isthmus 4-8 /u ; thickness 8-16 /u. 

 (PL LXI, fig. 33). This form we have observed from Thursley 

 Common, Surrey. 



(3) A form in which the cells are slightly retuse in the 

 middle of each apex; cell-wall delicately punctate; length 

 32 /; breadth 27'5ju; breadth of isthmus 5'5 /u (PI. LXI, 

 fig. 29). This form we described and figured in f Alg. 

 N. Ireland/ 1902, p. 40, t. 2. f. 10. It might stand as " var. 

 ellipsoideum forma retusa" It was found at Grlenties and in 

 Lough Anna, Donegal. 



Var. Gartanense West & G. S. West. (PL LXI, fig. 27.) 



C. contractum Kirchn. var. Gartanense West & G-. S. "West, Alg. X. Ireland, 

 1902, p. 41, t. 2, f. 9. 



Cells larger and usually a little longer ; semicells 



o / 



at the middle of the apex abruptly retuse-emarginate. 



Length 68 //, ; breadth 43-44 p. ; breadth of isthmus 

 10 p.. 



IEELAXD. Lousfli Gartan, Donegal ! 



