VIII] OF CERTAIN DIATOMS 617 



When the partitions A, F meet the equatorial cells on the hither 

 side of the partitions a, /, then, obviously, the large cell R is an 

 irregular hexagon, and is in contact with two equatorial cells only; 

 such an arrangement is said to define the genus Orthoperidinium. 

 When A, F happen to fall on the farther sides of a,/, the large cell 

 has eight sides, and is in contact with four equatorial cells ; we have 

 the new genus Paraperidinium. When A falls within a, but F falls 

 beyond /, we have the genus Metaperidinium. There remains one 

 alternative case, the converse of the last, to which the systematists 

 have not given a name. 



The same physical phenomenon occurs at the opposite pole of the 

 disc, where the partitions C, D may fall within or without, or one 

 within and one without the positions of c, d\ where, in other words, 

 the intercalated cell CD is in contact with one, with three, or with 

 two equatorial cells. Jorgensen, seeing these three types occurring 

 both among the Orthoperidinia and the Metaperidinia, draws the 

 conclusion that this character is more primitive, or more ancestral, 

 than that by which Ortho- and Metaperidinia are separated from 

 one another, a phylogenetic deduction concerning which topology 

 has nothing to say*. Within the restricted genus Peridinium we 

 have at present two sub-genera and seven sub-groups of these, this 

 being the number of the 64 possible arrangements so far recognised 

 and named. These may have a certain constancy or stability ; and 

 trivial as their differences may seem to the physicist, they may still 

 be worth the naturalist's while to study and record. 



Ano'ther case, geometrically akin but biologically very different, 

 is to be found in the httle diatoms of the genus Aster olampra, and 

 their immediate congenersf. In Asterolampra we have a little disc, 

 in which we see (as it were) radiating spokes of one material alter- 

 nating with intervals occupied on the flattened wheel-like disc by 

 another (Fig. 266). The spokes vary in number, but the general 

 appearance is in a high degree suggestive of the Chladni figures 

 produced by the vibration of a circular plate. The spokes broaden 

 out towards the centre, and interlock by visible junctions, which 



* E. Jorgensen, Ueber Planktonproben, Svenska Hydrogr. Biol. Komm. Skrifter, 

 IV, 1913. 



t See K. R. Greville, Monograph of the genus Aster olampra ^ Q.J. M.S. viii, 

 (Trans.), pp. 102-124, 1860; cf. ibid. (n.s.). ii, pp. 41-55, 1862. 



