20 1', T. CLEVE, NUTEt^ ON SOME ATLANTIC I'LANKTON-OKGANISMS. 



as will be scon from the figures, PI. VIII, fig. 6 9. Of these the fig. 6 represents ;i 

 .speeimen of A. Iwptactis (23'i of May 1898 58 48' N., 1845' W., Ostenfeld's collection), 

 the fig. 9 a very small specimen of A. atlanticxis from the same gathering. Tlie fig. 8 

 represents a specimen of ordinary size from Davis Strait and the fig. 7 a specimen from 

 the coast of Portugal, the largest I have seen. 



The specific difference may be seen from the follo\\ing comparison: 



A. heptactis. A. atlanticus. 



Size 0,05 to 0,1 millim. 0,o25 to 0,06 millim. 



Radial proportion of the umbilical 



space and the limbus 1:3. 1:2. 



Alveoli in 0,oi millim 6. 12. 



Geographical distribution .... 26 S. 63 N. 46 N. 78 N. 



Plankton-type Styli-plankton. Cheto- and triclw-pJankton. 



That A. heptactis and A. atlanticus are different species cannot be doubted, but 

 the question whether A. atlanticus and A. Broohei are identical or not, is more difficult to 

 decide ^\'ithout comparison of original specimens. Gkeville's figure of A. Brookei from 

 Kamtchatka (Trans. Mic. Soc, \o\. VIII, PI. IV, fig. 18) has 10 alveolate fields, else it 

 seems to agree well, but as the minute structure has not been accuratelv described, the 

 identification is impossible. It Avould be of a great interest to settle that question.*) 



Chictoceros difficilis. N. Sp. 

 riiitc VIII. Fig. 1618. 



Chains loose, of quadrate cellules, separate by large rectangular or subhexagonal 

 foramina, half as long as the cellules. Cellules thin-walled, in sagittal-longit. i)Osition 

 (juadrate -with fiat valves. Cell-contents (as far as could be ascertained on alcohol-preserved 

 specimens) with only one chromatophore. All a-\vns very delicate. Endocysts \n the 

 middle of the cellules, biconvex, smooth, but with a periplierical row of small puncta. 



Longitudinal axis 0,oo.-. to 0,oo8. Sagittal axis 0,oo8 to 0,oi. Endocysts 0,oo7 to 0,oi 

 millim. Sterile specimens resemble C. balticus Cl. (Bih. till Kongl. Sv. Vet.-Akad. Handl., 

 V'ol. XXI 3, n:o 5, pag. 25), but the endocysts are different. 



This species was found in September 1899 at 59" N., 1 2' W. (temperature 11,8, 

 salinity 35,3?) together with forms that belong to tripos- and sira-plankton. 



Cbaetoceros longisetiis. N. Sp. 



Plate VII. Fig. 2.520. 



Chains composed of numcrnus cellules, stniiglii, Ihiii-walled, without foramina. 

 Cellules in longitudinal-sagittal position (juadrate, with flat valves. Awns arising from 



*) When this paper was in the jji'ess, I received samples from the southern Atlantic, which cnutained 

 aliuiulantly A. Hookeri Ehb. This antarctic form agrees perfectly with the boreal A. athmtirus, which name 

 consciiucntly must be exchanged for A. Hookeri. 



