T. T. OLEVB ON SOME FOSSIL MARINE DIATOMS. 1 7 ^ J 



T.M.S., 1871. Strangulomena, Greville, T.M.S. (1873), and a new 

 and undescribed genus from the Tegel, Pyrgodiscus, Kitton, MS. 



The genus Thaumatonema bears a long process, proceeding from 

 the centre of the circular valve, which afterwards becomes widely 

 forked. The apices of the forks articulate with those of the ad- 

 joining frustule. Valve (discoid) turgid, with a flattened centre. 



Pyrgodiscus (from Tlvpyos, a tower), has a conspicuous square, 

 tower-like elevation, bearing large spines on its sides and angles. 

 Valve orbicular, with a large square central elevation, armed with 

 long and stout spines ; margin with small processes placed on 

 elevations. Surface with irregular line of radiating puncta. The 

 processes are connected with the central elevation by smooth 

 radiating lines. 



Pyrgodiscus armatus, n.g. Kitton. End of the central 

 elevation flattened, with rows of small puncta, which do not reach 

 the centre, margins with four stout spines, below which and alter- 

 nating with them are four similar spines. Diameter of disc 'OS-'l 

 mm. (height of "tower" 2-r4 F.K.) The structure of the 

 valve seems to bear some resemblance to that of Polymyxus (?) 

 pulchellus, Grun. (in Van Heurck Sy., PI. 123, f. 5) and Tliauma- 

 tonema costatum, Grew (T.M.S. 13, PI. 8, f. 3). PI. 13, f. 13 a.b.c. 



Herr Thum has not yet succeeded in rinding an entire frustule ; 

 it is therefore uncertain whether the two valves are alike. I am 

 much inclined to believe that they are not. My reason for this is 

 that he has sent me a specimen, which seems to me to be either 

 the opposite valve of a frustule or an abnormity. It is certainly 

 nearly allied to P. armatus. The following is a description of 

 it : Valve more irregular in structure ; it has 5 spines and a slight 

 indication of a sixth, but which do not project from a central eleva- 

 tion. PI. 13, f. 13, d. 4 «°. 



Arachxoidisca Cl., N. Fam. 



Arachnoidiscus omatus, Ehb. (1849). A. Schm. Atl., PI. 73, 

 f. 4-6. Several corroded specimens. 



Stictodiscus Californicus, Grey., var. areolata, Grun. A. 

 Schm. Atl , PI. 74, f. 1. (1882). I have seen five specimens 

 agreeing well with the figure in the Atlas, taken from a somewhat 

 corroded valve from the Moron deposit. Diameter - 08-*l5 mm. 

 On uninjured specimens, the lines of marginal puncta split up into 

 2-3 rows of small puncta. 



