COSCIXODISCEJE '. ACTINOPTYCHUS. 



209 



differs in form from the others, and is sometimes specially con- 

 tinuous with the umbilicus (Plate I., fig. 4). The excentricity 

 of the other rays which is thus produced has been made the basis 

 of another Generic designation, Spatangidium ; but it may be 

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

 beautiful disks are for the most part obtainable from Guano, and 

 from Soundings in tropical and antarctic seas. From these we 

 pass on to the Genus Actinoptychus (Fig. 133), of which also the 

 frustules are discoidal in form, but of which each valve, instead of 

 being flat, has an undulating surface, as is seen in front view (b) ; 

 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 dif- 

 ference of aspect Fig. 133. 

 which the different 

 radial" divisions pre- 

 sent in Fig. 133, is 

 simply due to the 

 fact that one set is 

 out of focus whilst 

 the other is in it, 

 since the appear- 

 ances are reversed by 

 merely altering the 

 focal adjustment. 

 The number of radial 

 divisions has been 



considered a character of sufficient importance to serve for the 

 distinction of species ; but this is probably subject to vai-iation ; 

 since we not unfrequently meet with disks, of which one has (say) 

 8 and another 10 such divisions, but which are so precisely alike 

 in every other particular that they can scarcely be accounted as 

 specifically different. The valves of this genus also are very abun- 

 dant in the Infusorial earth of Richmond, Bermuda, and Oran 

 (Fig. 144, b, b, b) ; and many of the same species have been found 

 recently in Guano, and in the seas of various parts of the world. 

 The frustules in their living state appear to be generally attached 

 to Sea-weeds or Zoophytes. 



229. The Bermuda earth also contains the very beautiful form 

 (Plate I., fig. 3), which, though scarcely separable from Actinop- 

 tychus except by its marginal spines, has received from Prof. 

 Ehrenberg the distinctive appellation of Heliopdta (sun-shield). 

 The object is represented as seen on its internal aspect by the 



* See Greville in " Quart. Journ. of Microsc. Science," Vol. vii. 

 (1859 , p. 158, and in "Transact, of Microsc. Soc," Vol. viii. X. S. (I860', 

 p. 102, and Vol. x. (1862), p. 41 ; also "Wallich in the same Transactions, 

 Vol. viii. (1860), p. 44. 



Actinoptychus un<1v.latv.$ ; — A, side view 

 b, front view. 



