142 THE VOYAGE OF H.M.S. CHALLENGER 



Since then the generic designation given originally by Ehrenberg must be looked upon 

 as valid, it may further be noted that Pritchard and Ralfs 1 unite under the designation 

 of Actinocyclus ehrenbergii not less than one hundred and sixteen species which Ehren- 

 berg had established by giving special value to the radiating divisional lines, and had 

 named after all the gods of Olympus, afterstars, historical celebrities, gems, &c. At the 

 same time the species Actinocyclus moniliformis, Ralfs, 2 Actinocyclus raljsii, W. Sm., 3 

 Actinocyclus fulvus, W. Sm., 3 and Actinocyclus crassus, W. Sm., 3 are retained, while Actino- 

 cyclus panhelios, Ehrenb., is regarded as a doubtful species. Actinocyclus inter punctatus, 

 Bright., 4 and Actinocyclus subtilis, Greg., 5 are looked upon as the representatives of a 

 section whose discs are generally coloured and ornamented by radiating series of points, 

 while Actinocyclus tessellatus, Roper, 6 is regarded as the single type of a second section 

 embracing cellulated hexagonal discs without radiating lines. The Actinocyclus interpunc- 

 tatus, Bright., however, has no trace of an intramarginal pseudo-nodule, and, therefore, 

 cannot be regarded as belonging to the present genus, while Actinocyclus tesselatus, Roper, 

 possesses neither a pseudo-nodule nor any other characteristic which might authorise its 

 ascription to this genus. 



Rabenhorst, in his Flora Europaea Algarum Aquae dulcis et submarines, follows the 

 example of Kutzing in omitting the important generic character above referred to, but that 

 work must be admitted to be greatly deficient generally in its account of marine Diatoms. 



It follows, therefore, that although the presence of a pseudo-nodule is indispensable to 

 the conception of the genus Actinocyclus, the radiating lines or points which divide the 

 surface into compartments have a more or less accidental character, while among the 

 specimens collected by H.M.S. Challenger discoidal forms distinctly provided with intra- 

 marginal pseudo-nodules have been observed to be either finely and closely or sparsely 

 punctated, to possess discoid surfaces either with or without radiating fines, to exhibit 

 large radiating cells, or finally, to have a marginal pseudo-nodule and a disc minutely and 

 closely but irregularly punctated. Since then the essential conditions which separate one 

 organic form from another must be recorded, and since less essential characteristics become 

 more and more extended as the discovery of new and kindred forms goes on, the original 

 definition given by Ehrenberg may now be modified in the following manner : — Frustula 

 simplicia, disciformia, punctulata vel cellulosa, plerumque nonnulis lineis radiantibus 

 distincta, circulari pseudonodulo intramarginal i instructa. 



1 Pritchard, op. cit., p. 834. 2 = Actinocyclus ternarius, Ehrenberg, Mikrogeologie, pi. xxii. fig. 9. 



,. 3 = Eupodiscus ralfsii, Eupodiscus fulvus, and Eupodiscus crassus respectively. These forms, according to 

 Smith, probably belong to the genus Actinocyclus, Ehrenb., but as he has " limited that genus to frustules with 

 undulated valves," he has found it necessary to place these apart. Their position in Eupodiscus he regards, 

 however, as doubtful since " the process in all is rather a pseudo-nodule than a projection from the surface 

 of the valve." Smith, Synop. Brit. Diat., vol. i. p. 24, vol. ii. p. 86. 



4 Micr. Jouni., vol. viii. p. 94, pi. vi. fig. 17. 



5 = Eupodiscus subtilis, Gregory, Diatoms of the Clyde, p. 29, pi. iii. fig. 50. 



6 = Eupodiscus tessellatus, Micr. Journ., vol. vi. p. 19, pi. iii. figs, la and 16. 



