20 P, T. CLEVE, NOTE£ ON SOME ATLANTIC PLANKTON-ORGANISMS. 



as wiM be seen from the figures, Pl. VIII, fig. 6—9. Of these the fig. 6 represents a 

 spociraen of A. heptactis (23 d of May 1898 58° 48' N., 18° 45' W., Ostenfeld's collection), 

 the fig. 9 a very small specimen of A. atlanticits from the same gathering. The 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 following comparison: 



A. heptactis. A. atlanticus. 



Size 0,05 to 0,i millim. 0,025 to 0,05 milliin. 



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 tricho-plankton. 



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

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

 decide without comparison of original specimens. Greville's figure of A. Brookei from 

 Kamtchatka (Träns. Mic. Soc, Vol. VIII, Pl. IV, fig. 18) has 10 alveolate fields, else it 

 seems to agree weU, but as the minute structure has not been accurately described, the 

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



Chretoceros difficilis. N. Sp. 

 Plate VIII. Fig. 16—18. 



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



fora mina, half as long as the cellules. Cellules thin-walled, in sagittal-longit. position 

 quadrate with flat valves. Cell-contents (as far as could be ascertained on alcohol-preserved 

 specimens) with only one chromatophore. All awns very delicate. Endocysts in the 

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



Longitudinal axis 0,oo5 to 0,oos. Sagittal axis 0,oos to 0,oi. Endocysts 0,oo7 to 0,oi 

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

 Vol. XXI X, n:o 5, pag. 25), but the endocysts are difterent. 



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

 salinity 35,37) together with forms that belong to tripos- and sira-plankton. 



Chsetoceros longisetns. N. Sp. 

 Plate VII. Fig. 25—20. 



Chains composod of riumerous cellules, straight, thin-walled, without foramina. 

 Cellules in longitudinal-sagittal position quadrate, with Hat valves. Awns arising from 



*) When this )>a]»er was in the press, I received samples from the Bouthern Atlantic, which contained 

 abundantly A. HooTceri Ehb. This antaretie form agrees perfectly witli the boreal A. atlanticus, which nanic 

 consequcntly must be exchanged for A. Hookeri. 



