128 THE VOYAGE OF H.M.S. CHALLENGER 



Dr James Rae, E.N., and kindly placed at my disposal in connection with my present 

 work, I found this singular frustule, wliick, although brought from the interesting and 

 rich deposits of St Monica in California, I have deemed worthy of being figured in order 

 to convey a clear idea of its remarkable structure. The large disc possesses six compart- 

 ments, and the valve is delicately granulated in a quincuncial manner. The large cellules 

 or areolae possess a subquadrate form — a character which is not represented in any other 

 type. At the margin numerous large denticules occur ; these are irregularly distributed, 

 and project from the inner part of the margin so as to cause the frustule to assume the 

 appearance of an irregularly toothed wheel. 



The specific name which has been given is in honour of its discoverer, Dr James Eae. 



Actinoptychus erosus, n. sp. (Plate VII. fig. 8.) 



Frustulum disciforme in decern septa a tot lineis lsevibus divisum, et decussatim 

 granulatum ; granulis prope marginem et alibi quandoque cessantibus ; area centrali 

 hyalina rotunda. In Atlantico meridionali. 



This form, which was collected in the South Atlantic, is divided into ten compart- 

 ments by ten smooth lines which do not quite reach the centre, this being occupied by an 

 umbilical hyaline area. The compartments are granulated in a quincuncial manner, except 

 in some small irregular spots, which are smooth, as if the valve had been eroded at the 

 places in question. The margin consists of a smooth narrow belt. 



The eroded-like aspect of the compartments has suggested the specific name. 



Omphalopelta (Ehrenb.), Cstr. 



Much confusion has hitherto existed in connection with the genera Actinoptychus and 

 Omphalopelta, both of which were instituted by Ehrenberg. The definition of the latter 

 genus, as given by Pritchard in his History of the Infusoria (p. 841), is as follows: — 

 " Frustules simple, disciform ; disc cellulose or punctate, divided by imperfect septa into 

 cuneate rays ; centre hyaline ; spines one to each compartment." 



Among the four species given by Pritchard under this genus, Omphalopelta cellulosa, 



Ehrenb., and Omphalopelta areolata, Ehrenb., 1 have marginal spines in three of the 



alternate compartments, while Omphalopelta punctata, Ehrenb., is said to have obsolete 



spines. But it is difficult to understand how forms possessing such a structure can be 



ascribed to this genus. With respect to the so-called Omphalopelta punctata, Ehrenb., 



there can be no doubt that it should rather be designated Actinoptychus punctatus, as 



it coincides with all the requirements of that genus, whde in the case of the other two 



above-mentioned species, either the original definition given by Ehrenberg must be 



modified, or they must be enrolled in a new genus. 



1 Ehrenberg, Mikrogeol., pi. xxxv. A 18, fig. 12. = Actinocyclus areolaris, Brightwell, Micr. Journ., vol. 

 viiL p. 93, pi. v. figs, la and 16. 



