298 DIATOMACE.E '. COSCINODISCEjE. 



especially the Infusorial earth of Richmond in Virginia, of Bermuda, 

 and of Oran, as also in Gruano. Each frustule is of discoidal shape, 

 being composed of two nearly flattened valves, united by a hoop ; 

 so that, if the frustules remained in adhesion, they would form a 

 filament resembling that of Melosira (Fig. 131). The regularity 

 of the hexagonal divisions on the valves renders them beautiful 

 microscopic objects ; in some species the areolae are smallest near 

 the centre, and gradually increase in size towards 'the margin ; 

 in others a few of the central areolae are the largest, and the rest 

 are of nearly uniform size ; while in others, again, there are 

 radiating lines formed by areolae of a size different from the rest. 

 Most of the species are either marine, or are inhabitants of brackish 

 water ; when living they are most commonly found adherent to 

 Sea-weeds or Zoophytes ; but when dead, the valves fall as a sedi- 

 ment io the bottom of the water. In both these conditions, they 

 were found by Prof. J. Quekett in connection with Zoophytes which 

 had been brought home from Melville Island by Sir E. Parry ; and 

 the species seemed to be identical with those of the Richmond 

 earth. The Genus Actinocyclus* 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 Gruano from Ichaboe. Allied to the preceding are the 

 two Genera Asterolampra and Asteromphalus, both of which have 

 circular disks of which the marginal portion is minutely areolated, 

 whilst the central area is smooth and perfectly hyaline in appear- 

 ance, but is divided by lines into radial compartments which 

 extend from the central umbilicus towards the periphery. The 

 difference between them simply consists in this ; that in Astero- 

 lampra all the compartments are similar and equidistant, and the 

 rays equal (Plate i., fig. 2) ; whilst in Asteromphalus two of the 

 compartments are closer together than the rest, and the enclosed 

 hyaline ray (which is distinguished as the median or basal ray) 



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

 the genus Actinocyclus to the forms originally included in it by Ehren- 

 berg, and to restore the genus Actinoptychus of Ehrenberg, which had 

 been improperly united with Actinocyclus by Profs. Kiitzing and 

 W. Smith. 



