COSMARIUM. 247 



apex 18-24 ju,; breadth of isthmus 12-21 p; thickness 

 2 7-46 p.. 



SCOTLAND. Rhiconich, Sutherland ! 



Geogr. Distribution. Austria. Poland. Ceylon. 

 Sumatra. Central Africa. Brazil. Australia. United 

 States. 



C. binum has a wide distribution and exhibits much varia- 

 bility in size in different localities. It is somewhat rare in 

 temperate climates,, but it is the most frequent of the C. spe- 

 ciosum-group in the tropics. The larger forms possess more 

 marginal crenations than the smaller forms, but differ in no 

 other points, and as all intermediate states exist between those 

 with 17 and those with 26 marginal crenations per semicell, 

 we have not discriminated between them. 



One of the principal features of C. binum is its central 

 tumour, which consists of a large rounded inflation traversed 

 by a number of vertical ridges. Each ridge is notched at 

 regular intervals and thus cut up into a number of more or 

 less quadrangular granules. Beneath the tumour, and some- 

 what isolated from it, is a single (very rarely a double) series 

 of rounded granules. These granules are generally con- 

 spicuous, and we have regarded the combination of this basal 

 series of granules with the type of tumour just described as 

 the diagnostic feature of C. binum. 



C. binum should be compared very carefully with C. pul- 

 cherrimum and with C. subspeciosum. From the former it is 

 distinguished by its truncate apices, and by its larger, dif- 

 ferently granulated tumour, with the distinct isolated series 

 of basal granules. From the latter it differs in the broader 

 truncate apices, with more crenations, in the more regularly 

 binate granulation of all the marginal crenations, and in the 

 nature of its central tumour. 



There is a smooth area between the central tumour and the 

 radial granules, and also in the centre of the apex (as seen in 

 vertical view). 



211. Cosmarium speciosum Lund. 

 (PI. LXXXIX, figs. 1-3.) 



Cosmarium speciosum Lund. Desm. Suec. 1871, p. 34, t. 3, f . 5 ; Wolle, 

 Desm. TJ. S. 1884, p. 87, t. 19, f. 7-8 ; Cooke, Brit. Desm. 1887, p. 117, 

 t. 41, f . 1 ; Boldt, Desm. Gronland, 1888, p. 19 ; West, Alg. X. Yorks. 

 1889, p. 293 ; Alg. N. Wales, 1890, p. 290 ; Borge, Chlor. Norska Fin- 

 mark. 1892, p. 10 ; Eacib. Desmidyja Ciastonia, 1892, p. 386, t. 6, . 13 ; 

 Liitkem. Desm. Attersees, 1893, p. 554; Nordst. Index Desm. 1896, 



