6lO MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



the hexagons and the rings in the midst of them appear thickene 

 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. It is very desirable 

 that living forms of Coscinodisci should be carefully examined ; 

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

 truded through them. 



The genus Actlnocjjclus l closely resembles the preceding in form, 

 but differs in the markings of its valvular discs, which are minutely 

 and densely punctated or areolated, and are divided radially by 

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

 but interrupted. The discs 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 bv 

 Professor Ehrenberg on minute differences presented by the rays as 

 to number and distribution ; but since scarcely two specimens c.-in 

 be found in which there is a perfect identity as to these particulars, 

 it is evident that such minute differences between organisms other- 

 wise 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 Asterolampru and AsteromphcdubS, 

 both of which have circular discs of which the marginal portion is 

 minutely areolated, whilst the central area is smooth and perfectly 

 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 Ast&ro- 

 lampra all the compartments are similar and equidistant and the rays 

 equal, whilst in Aster omplial us (PI. I, fig. 3) two of the compartments 

 are closer together than the rest, and the enclosed hyaline ray (which 

 is distinguished as the median or basal ray) differs in form from 

 the others, and is sometimes specially continuous with the ximbilicus. 

 The eccentricity thus produced in the other rays has been made the 

 basis of another generic designation, SpatangidiiJ/ni ; but it may be 

 doubted whether this is founded on a valid distinction. 2 These 

 beautiful discs are for the most part obtainable from guano, and 

 from soundings in Ir.ipical and antarctic seas. From these we pass 

 on to the genus Actinoptychus (fig. 456), of which also the frustules 

 are discoidal in form, but in which each valve, instead of being flat, 

 has an undulating surface, as is seen in front view (1$), giving to the 

 side view (A) the appearance of being marked by radiating bands. 

 Owing to this peculiarity of shape, the whole surface cannot 

 be brought into focus at once except with a low power : and the 



1 The Aiitlinr rnnrtirs with Mr. Ralfs in thinking it pivt'eralile to limit the genus 

 Ictinocyclus to the forms originally included in it by Kln-eulierj,', and to restore the 

 f^enus Actinoptychus oi Khivnlier;.', \vliich had lieeii improperly united \viti\Actino- 

 <7/<7//\ liv Professors Kilt/ing and W. Smith. 



- Sec CJreville in < t hmr/. -lui/ni. Microsr. Sto'euce, vol. vii. 1859, p. 158; and in 

 'I'miix. .1/Yovi.sr. Soe. vol. viii. n.s. IftliO, p. lll-J. nnd vol. \. isil-j, p. 41 ; also Wallieh 

 in the same Transactions, \ol. \iii. iNtill. p. 44. 



