XANTHIDIUM. 69 



From Lougli Anna and near Lough Magrath in Donegal, 

 Ireland, a form of X. antilopseum was observed which wo 

 have referred to tin's variety as "forma spinis longioribus ; 

 tumore central! prominenti et supra tumorem cum scrobiculis 

 numerosis ' (dde W. & II. S. West, ( Alg. N. Ireland/ 1902, 

 p. 31). Length without spines 39 /LI, with spines 54 ^u; breadth 

 without spines 41 JJL, with spines 65 ju ; breadth of isthmus 

 ^ ; thickness 25 /i. (PL CIX, fig. 6.) 



Var. hebridarum AY. & G. S. West. (PI. CIX, fig. 7 ; 



PL CX, figs. 1, 2.) 



X. antilop;mrn var. hebridarnm W. & G. S. West, Further Contrib. Fresh vv. 

 Plankton Scott, Lochs, 1905, p. SCO, t. 7, f. 21 ; Comp. Study Plankton 

 Irish Lakes, 1906, p. 85 ; British Fresh\v. Phytoplankton, etc., 1909, 

 p. 180. 



Semicells with three spines on each lateral margin, 

 all of which are disposed in a median vertical plane, 

 one spine only inserted at the upper angles, the other 

 two approximated near the lateral angles; central 

 area consisting of a small obtusely mamillate pro- 

 tuberance around which is grouped in various ways a 

 number of minute scrobiculations. 



Length without spines 46 50 /A, with spines 61-71 /x ; 

 breadth without spines 42-49 JJL, with spines 69-82 JJL \ 

 breadth of isthmus 12-14 jit; thickness 31 /x, 



SCOTLAND.- -Plankton of two lochs in Inverness, two 

 in Ross, three in Sutherland, six in Lewis, Outer 

 Hebrides, and three in Harris, Outer Hebrides ! 



IRELAND.- -Plankton of several small lakes between 

 Clifden and Roundstone, Gralway ! 



Instead of the normal pair of spines at each upper angle 

 of the semicells, there is in this variety only a single spine, 

 and the pair of spines which should be attached side by side 

 at the lateral angles are here separated some distance apart, 

 and placed more or less vertically over each other. The 

 semicells are therefore not so angular as in typical X. anti- 

 ,lop&um. The length of the spines is variable, and examples 

 are sometimes met with in which the spines are to some 

 extent reduced and not strictly disposed in one vertical plane. 

 (vide PL CIX, fig. 7.) 



