MICROSCOPIC FORMS OF VEGETABLE LIFE. 291 



lower layer is nearly transparent, and little conspicuous when seen in bi- 

 sulphide of carbon, except, as shown in the figure, when the framework 

 of the hexagons, and the rings in the midst of them, appear thickened 

 and more refractive. In both layers the balance of observations tends 

 to the belief that the hexagons have no floors, and are in fact perforated 

 by foramina like those of minute Polycystina. The cells formed by the 

 hexagons of the upper layer are of considerable depth; those of the 

 lower layer are shallower. In both layers fractured edges show the hex- 

 agon-frames to be the strongest parts; and in neither has Mr. Stephen- 

 son been able to detect any broken remnants of floors, which might be 

 expected to be visible with high powers if they existed at all. If further 

 observations should confirm Mr. Stephenson's belief that Coscinodisci 

 are perforated by numerous foramina, a similar structure will be sought- 

 for in other Diatoms, and the views of naturalists as to the character of 

 the group may be materially modified. At present the chief difference 

 in minute structure that has been recognized, may be seen by comparing 

 the apparently simple beading of Pleurosigma with the hexagonal for- 

 mations in Coscinodiscus, etc. ; but a far more important divergence will 

 have to be considered, if some Diatom- valves have a multiplicity of fora- 

 mina, and others either none, or only a few at certain spots. It is very de- 

 sirable that living forms of Coscinodisci should be carefully examined; 

 since, if they really have foramina, some minute organs may be protruded 

 through them. 



290. The genus Actinocyclus 1 closely resembles the preceding in 

 form, but differs in the markings of its valvular disks, which are 

 minutely and densely punctated or cellulated, and are divided radially 

 by single or double dotted lines, which, however, are not continuous but 

 interrupted (Plate I., Fig. 1). The disks are generally iridescent; and, 

 when mounted in balsam, they present various shades of brown, green, 

 blue, purple, and red: blue or purple, however, being the most frequent. 

 An immense number of species have been erected by Prof. Ehrenberg on 

 minute differences presented by the rays as to number and distribution; 

 but since scarcely two specimens can be found in which there is a perfect 

 identity as to these particulars, it is evident that such minute differences 

 between organisms otherwise similar are not of sufficient account to serve 

 for the separation of species. This form is very common in guano from 

 Ichaboe. Allied to the preceding are the two genera Asterolampra and 

 Aster omphalus, both of which have circular disks of which the marginal 

 portion is minutely areolated, whilst the central area is smooth and per- 

 fectly hyaline in appearance, but is divided by lines into radial compart- 

 ments which extend from the central umbilicus towards the periphery. 

 The difference between them simply consists in this; that in Asterolampra 

 all the compartments are similar and equidistant, and the rays equal 

 (Plate i., Fig. 2); whilst in Asteromplialus two of the compartments are 

 closer together than the rest, and the inclosed hyaline ray (which is dis- 

 tinguished as the median or basal ray) differs in form from the others, 

 and is sometimes specially continuous with the umbilicus (Plate I., Fig. 

 4). The eccentricity thus produced in the other rays has been made the 

 basis of another generic designation, Spatangidium ; but it may be 



1 The Author concurs with Mr. Ralfs in thinking it preferable to limit the 

 genus Actinocyclus to the forms originally included in it by Ehrenberg, and to 

 restore the genus Actinoptychus of Ehrenberg, which had been improperly united 

 with Actinocychis by Profs. Kiitzing and W. Smith. 



