REPORT ON THE DIATOM ACE^E. 141 



Cyclotella fimbriata, n. sp. (Plate XVII. fig. 16.) 



Valvis discoidalibus ; area grandiuscula umbilicali bullato-rugosa, a qua stria) dense 

 radiantur, ad marginem fimbriam sirnulantes. In Oceano Pacifico. 



This Diatom was found in a soundiug made in the Pacific Ocean at lat. 35° 41' N., 

 and long. 157° 42' K, at a depth of 2300 fathoms. It is a small but elegant little disc, 

 adorned with very delicate radiating striae which diverge from the perimeter of a large 

 umbilical arch, which is rugged rather than granulated. The radiating striae form, near 

 the margin, a series of very small arches or elegant fimbria} — a circumstance which has 

 suggested the specific name of this new and interesting Diatom. 



Actinocyclus, Ehrenb. 



This genus was established by Ehrenberg and has been defined in the following 

 manner : — "Frustules simple, disciform, disc minutely and densely punctated or cellulose, 

 generally divided by radiating single or double dotted lines and having a small circular 

 hyaline intramarginal pseudo-nodule." 



In the review of the genus given by Pritchard in his History of the Infusoria (p. 833), 

 the confusion which has been observed in connection with it is ascribed partly to Kiitzing, 

 who included a few species of Actinoptychus, and partly to Smith, who, in his classic 

 Synopsis of the British Diatomaceae, named the genus Eupodiscus, a circumstance which 

 led both Gregory and Roper into error. The real ground of the confusion introduced by 

 Kiitzing, however, consisted not so much in the fact of his having embraced some of the 

 Actinopychi, as in the circumstance of his having falsified the definition given by Ehrenberg, 

 and in having replaced it by a new and substantially different definition, which ran as 

 follows : — " Actinoptychus: Individua solitaria, libera; lorica bivalvis disciformis cellulosa ; 

 cellulae radiis septisque internis radiantibus pluribus interruptae." 



It is manifest that Ehrenberg regarded the condition of the punctated or cellulose disc 

 with its intramarginal pseudo-nodule as the chief and essential characteristic, inasmuch as 

 he makes use of the word generally in speaking of the more or less punctated radiating 

 lines, while Kiitzing notes merely the " cellulae radiis pluribus laevibus interruptae" 

 without making the slightest mention of the pseudo-nodule. It cannot, however, be 

 denied that the pseudo-nodule constitutes a morphological factor of sufficient import- 

 ance to be of generic significance, so that it is necessary either to regard, like Ehren- 

 berg, that structure as an essential characteristic of the genus Actinocyclus, or to institute 

 a new genus to embrace those cellulated discs which only present radiating lines with 

 interruptions. In the latter case such lines should not be the means of detaching such 

 discoidal forms from the genus Coscinodiscus, of which Kiitzing has given the following 

 definition : — " Individua solitaria, libera, lorica bivalvis sdicea in latere secundario disci- 

 formis cribrata, sepimentis interioribus radiantibus nullis." 



