SYSTEMATIC ACCOUNT 259 



Chromatophores : numerous cocciform bodies lying close to the margin of the valve. 

 Diameter of valvar portion 200-220/A ; length of bristles usually 70-76/x. 



A weakly siliceous species, seldom found in large quantities, but having a wide dis- 

 tribution in tropical seas. Oceanic, favouring a low salinity. Common in the Indian 

 Ocean. Observed most frequently in material from around Madagascar. 



Observed at Sts. 440, 1373, 1575, 1583, 1586; WS 630, 631. 



Genus Actinocyclus Ehrenberg 

 Ehrenberg, 1838 



Ehrenberg (1838, p. 171) established the genus Actinocyclus, with the following 

 characters: "Animal e familia Bacillariorum, liberum, lorica simplici, bivalvi (silicea), 

 snbcylindricam (disciforme), septis intends radiantibus pluribus, divisione spontanea im- 

 perfecta cateniforme." 



Two species were described (p. 172) in the following manner: 



210. Actinocychis senarius sechszellige Strahlendose, Tafel 21, fig. 6. A. lorica celhdosa, 

 disciformi, radiis internis cellulisque senis. 



211. Actinocyclus octonarius, achtzellige Strahlendose, Tafel 21, fig. 7. A. lorica cellu- 

 losa, disciformi, radiis internis cellulisque octonis. 



Ehrenberg did not state which of the above he regarded as the type of the genus, and 

 the generic description does not accurately describe either of them, but may apply 

 equally well to both. It is quite likely that Ehrenberg did not appreciate fully the 

 structure of the organisms he described, but it is evident that subsequently he recog- 

 nized certain differences between the two species. The illustration provided in his fig. 6 

 shows a small circular valve, regularly and clearly divided into six compartments by de- 

 finite radiating lines. The segments are alternately light and dark, and filled with granules. 

 Fig. 7 on the same plate shows a larger discoid valve, divided into eight compartments. 

 The segments are not alternately light and dark, and are filled with fine dots. 



Bailey (1842, pp. 93, 94, 96) made use of the genus Actinocyclus Ehrenberg, to which 



he referred a number of specimens found fossil from Richmond, Virginia, and produced 



figures of Actinocyclus (pi. 2, figs. 9-1 1) which are identical with that of Ehrenberg 



(1838, pi. 21, fig. 6). Bailey, however, did not make any specific determinations. 



Referring to his own work (1842, p. 94) he made the following note: 



Note, October 10th, 1841. Since the above was ready for the press, I have seen in the appendix 

 to Pritchard's History of Infusoria, living and fossil, some interesting statements of recent discoveries 

 by Ehrenberg, with reference to the genera of Actinocyclus and Coscinodiscus . . . (p. 96). Of the 

 genus Actinocyclus, Ehrenberg describes several new species. . . . Several of these species have no 

 partitions, but have surfaces marked with minutely punctate rays. . . . 



Bailey here referred to an appendix to Pritchard (1841), wherein are described seven 

 species of Actinocyclus. At this point it is clear that Ehrenberg had included in the 

 genus- Actinocyclus, two distinctly different forms. Ehrenberg himself was aware of this, 

 and feeling that a separation ought to be made, introduced the subgeneric heading 

 Actinoptychus, yet undescribed, in the following manner " Actinocyclus {Actinoptychus) 



