569 



year. Perhaps it is lliis species and not li. delicatula whicfa is cha- 

 racteristic of the plankton from Iceland (according to Granj. 



23. R. Stolterfothii, Perag., Diat. de Villefranche, p.90, PL 6, fig.4 I. 



A teraperate, neritic species which occurs in the southern part uf the 

 North Sea and still further south, only found twice in the planklon from 

 July (a few specimens . 



24. R. Shrubsolii Cleve, New Diatoms, Kgl. Vetensk. Akad. 



Handl., Bd. 18, No. 5, 1881, p. 26. 



Common in the plankton around the Færoes, found all the year 

 round, predominanl Ironi July November, especially in July— August. 

 Optimum of temperate 9°- 11 °C. [See tablcs.] 



It is a temperate, neritic form, which in the Atlantic Ocean 



occurs along the coast of Europe from Norway southwards. 



25. R. obtusa Hensen, V ter Ber. Komm. in Kiel, 1887, p. 86, 

 PL 5, lig. 41; R. alata var. truncata Gran., Protophyta, 1897, p. 6, 

 PL 4, figs. 6, 7. 



A form ratlier common occurs in the plankton from the Fær- 

 oes which I believe to be identical with R. obtusa of Hensen and 

 R. alata, var. truncata Gran. I have given a figure of 

 it showing the form of the calyptra. It will easily be 

 seen that the species comes near to R. alata, and I 

 should think that Gran is right in considering it to 

 be a mere variety. The form of the calyptra is not 

 alvvays the same, sometimes specimens occur which 

 approach the form ligured by Gran (Bemerk., 1900, 

 lig. 22) in having the calyptra elongated. 



Cleve regards (Addit. Notes on the season. Distrib. 

 of All. Plankton Organisms, Goteborg 1902) the R. inermis 

 of Castracane to be identical with this species, but ceU> 8howln * the 



form of the ca- 



I think he is wrong in doing so. lyptra (*>% 



It occurs from March to September, and in most 

 years rather sparingly, but predominated du ring May— June and August 

 of 1898 and 1902. It is a boreal, oceanic species. Optimum of tempe- 

 rate 6°— 7°C. [See tables.] 



26. R. semispina Hensen, V tei Ber. Komm., 1887, p. 84, PL 5, 

 fig. 39 a, b. 



A borcai, oceanic species, rare around the Færoes, only a few speci- 

 mens were found in samples from March, June, July, August and 

 November. 



27. R. styliformis Btw., Mier. Journ., 1858, p. 96, PL 5, fig. 

 5 a, b, c, d. 



Fig. 125. Rhizo- 

 solenia obtusa 

 Hensen. Two 



viewsof the same 



