112 THE VOYAGE OF H.M.S. CHALLENGER.] 



The specimen figured was obtained in a sounding made near the Azores in the 

 Atlantic Ocean. 



Stictodiscus, Grev. 



This genus was established by Greville in a communication read on 12th March 1861, 1 

 and he classed in it Discoplea (?) rota 2 and Discoplea rotula, 3 Ehrenberg, as well as 

 Actinoptychus dives, i which, as already suggested by Ehrenberg, might be generically 

 associated with Cyclotella rota and Cyclotella rotula. The words of Ehrenberg are (under 

 his definition of Discoplea (?) rota) : " Proxime ad Actinoptychum divitem in Grsecia fossilem 

 accedens forma, et cum ea forsan, et cum sequente (Discoplea (?) rotula) in peculiari 

 genere reponenda." 



To this genus Greville at this time ascribed six species, and later Kitton 5 made us 

 acquainted with Stictodiscus crozieri. Grunow 6 added his Stictodiscus angulatus in the 

 Typenplatten of Moller, while Cleve 7 announced his Stictodiscus novarce in a paper com- 

 municated to the Eoyal Swedish Academy of Science on 15th September 1880. 



The definition of the genus Stictodiscus, as given by Greville, ran as follows : — 

 "Frustules simple, discoid, divided by radiating lines into numerous plicate compart- 

 ments. Lines not reaching the centre. Compartments furnished with conspicuous trans- 

 parent, pore-like points. (In the four typical species, large scattered puncta also occupy 

 the blank central portion of the disc.)" 



Among the materials collected by the Challenger Expedition a considerable number 

 of specimens of Stictodiscus has been observed, but these were only found in a few locali- 

 ties, the most noteworthy of which are the Japanese coast and Zebu in the Philippine 

 Islands. The examination of these specimens has convinced me that the structural 

 peculiarities, and particularly the folded compartments, are somewhat variable and do not 

 always afford sufficient ground for establishing new species. All the forms may be reduced 

 to two principal sections, namely (1.) radiate Stictodisci, in which the discs possess com- 

 partments and the folds are simply radiating ; and (2.) radiato-areolate Stictodisci, in 

 which the discs have radial folds which proceed from the circumference, but soon bifurcate 

 or are interrupted so as to bound numerous polygonal areas. It is to be noted, however, 

 that this classification, though of use in facilitating the description of forms so similar to 



1 Micr. Journ., n. s., vol. i. p. 39. 



2 Monatsber. d. Jc. Mead. d. Wiss. Berlin, 1844, p. 202; MikrogeoL, pi. xxxv. A 22, fig. 6. 



3 Ehrenberg, MikrogeoL, pi. xxxv. A 22, fig. 7. 



4 Ehrenberg, MikrogeoL, pi. xix. fig. 12 ; Ralfs in Pritchard's History of the Infusoria, 4th ed.,'p. 840. 



5 Micr. Journ., 1873, pi. xxxviii. fig. 2. 



6 In Schmidt's Atlas (explanation of plate Ixxiv. figs. 26-30), a note is appended to Grunow's Pseudosticto- 

 discus angulatus to the following effect : " Grunow bemerkt dazu; die beiden Schalen seien nicht ganz gleich, 

 die flachere habe keine Rippen, und immer in der Mitte einen rudimentaren Stachel, der in meinen Zeichnungen 

 fehle; die Ecken etwas vorspringend, darum den Biddulphien sich nahernd." <1 



7 Kongl. SvensJc. Vetensk.-Akad. Handl., 1880, Bd. xviii. No. 5, p. 21, pi. v. fig. C6. 



