REPORT ON THE DIATOMACE/E. 85 



Corethron, 1 n. gen. 



The new forms upon which this genus is established are exclusively Antarctic. They 

 have a more or less cylindrical form, and are terminated by hemispherical surfaces, the 

 base being surrounded by a corona of awns, which are more or less thick and long, and are 

 smooth or thorny. Although from the rarity of these interesting organisms it was not pos- 

 sible to determine their siliceous or non-siliceous character by means of acids, this determina- 

 tion was made by the application of strong heat for a protracted period. This produced 

 no visible change on their form, so that their diatomaceous nature could not be doubted. 



Although the presence of long awns radiating round the valves might affiliate the 

 present organisms to the genus Bacteriastrum, the two groups of forms differ very 

 notably in other respects. Thus in the former there is as yet no proof that the frustules 

 are arranged in series, while their general form is cylindrical or cylindroidal — one axis 

 being much longer than the other — and the awns are never tortuous or dichotomous. 

 But these characteristics are not wanting in the genus Bacteriastrum, hence the new 

 series of forms, which unquestionably have certain affinities to Chsetoceros, must be 

 regarded as forming a transition to the tubulate Rhizosolenise. 



The name that has been proposed for the genus has reference to the " broom "-like 

 appearance of the frustules, and the following generic definition may be given : — Frustula 

 cylindrica, libera (?) ; valvis convexis, setarum radiantium corona cinctis. 



According to the character of the connecting zone and awns this genus may be divided 

 into two sections, embracing — 



A. Forms with simple smooth connecting zones and smooth awns. 



B. Forms with complex annulate connecting zones and echinated awns. 



Section A. Connecting zones simple and smooth ; awns smooth. 



Corethron criophilum, n. sp. (Plate XXI. fig. 14.) 



Forma longe cylindrica, valvis producto-convexis, setis tenuissimis. In Antarctico ad 

 glacies impervios. 



This long and perfect little cylinder has a longitudinal axis, which bears to its diameter 

 the ratio of 14 to 1. The awns are long and very delicate, smooth and radiating in the 

 same direction at the two extremities. The two valves are extremely convex. This 

 type occurs not unfrequently at the Antarctic ice barrier, and on floating fields of ice. 



Corethron criophilum, n. sp., var. nov. (Plate XXI. figs. 12 and 15.) 



Probably the frustules here shown only represent two varieties of the preceding species. 

 Both differ, however, from the latter (1.) in having the longitudinal axis shorter and 



1 xigriQw, broom. 



