78 BRITISH DESMIDIACE^E. 



ENGLAND.- -Brothers' Water and Deepdale, West- 

 moreland ! Bog two miles S. of Clapham, W. Yorks ! 

 Strensall, N. Yorks (Turner). Gloucester (Ralfr).. 

 Piltdown Common, Sussex (Jenner). Penzance (Red/*) 

 and near St. Just !, Cornwall. 



WALES.- -Llyn Padarn !, and Capel Curig ! (Coolie Jj- 

 Witts), Carnarvonshire. 



SCOT LAND.- -Aberdeen, Kincardine, Forfar, Perth!, 

 Dumbarton. (Roy $ Bissett). Eenfrew ! 



IRELAND.- -Dublin and Wicklow (Archer). 



Geogr. Distribution.- -France. Germany. Italy. 

 Norway. Sweden. Bornholm. 



X. JBrebissonii is an uncommon species wliicli is very local 

 in its distribution. The form of its semicells and the number 

 of its paired spines are its distinguishing features. The spines 

 exhibit considerable variability in disposition, and the spines 

 of any pair may be inserted close together or far apart. 



Var. varians Ralfs. (PL CXII, figs. 2 and 4.) 



X. Brebissonii var. varians Ralfs, Brit. Desm. 1848, p. 113, t. 19, f. 2b,c,d 

 West, Alg. Engl. Lake Distr. 1892, p. 730. 



Semicells proportionately wider, on each side with a 

 small basal protuberance projecting into the sinus ; 

 spines generally more curved (often uncinate). 



Length without spines 61 JJL, with spines 82 ju, ; 

 breadth without spines 76 /*, with spines 108 p.; 

 breadth of isthmus 14-20/i; thickness 46 JJL. 



ENGLAND.- -Deepclale, Westmoreland! Piltdown 

 Common, Sussex (Jenner). Trewellard near Penzance, 

 Cornwall (Ralfs). 



10. Xanthidium aculeatum Ehrenb. 

 (PI. CXII, figs. 5-9.) 



Xanthidium aculeatum Ehrenb. in Abh. Acad. Wiss. Berlin, 1833, p. 318 [in 

 part] ; 1. c. 1836, t. 1, f. 11 ; Infus. 1838, p. 147, t. 10, f. xxiii b [figure 

 bad] ; Ralfs in Ann. Mag. Nat. Hist. 1845, p. 467, t. 12, f . 2 ; Hass. 

 Brit. Freshw. Alg. 1845, p. 360, t. 89, f . 3 [figure bad] ; Ealfs, Brit, 

 Desm. 1848, p. 113, t. 19, f. 1; Arch, in Pritch. Infus. 1861, p. 736; 

 Eabenh. Flor. Europ. Alg. Ill, 1868, p. 222 ; Lund. Desm. Suec. 1871, 

 p. 75, t. 5, f . 5 ; Kirchn. Alg. Schles. 1878, p. 155 ; Wolle, Desm. U. S. 

 1884, p. 92, t. 23, f . 10-12 [figures bad] ; Cooke, Brit. Desm. 1887, p. 130, 



