Dicdoinacese from the Caspian Sea. By A. Orunow. 689 



M. Petit's views in separating Podosira from Hyalodiscus on 

 account of the granular arrangement of the endochrome, and there- 

 fore remove this species from Hyalodiscus. The puncta are in 

 radiant and obHque hnes 13-15 in '01 mm. in the middle and 

 18 on the margin of the valve. The beginning of new rays of 

 puncta is mostly marked by a small blank space which appears 

 under certain focussing like a small dark spot. The centre of the 

 valve is more irregularly punctate, and occasionally a very small 

 irregularly bordered umbilicus is visible. Cahfornian specimens 

 reach a diameter of '24 mm. and have somewhat coarser puncta 

 (12-14 in "01 mm.), but it is impossible to separate them into 

 distinct species, and they can only be considered as a variety 

 " Calif orniea" of P. maxima. PL XXI., Fig. 2a {^^), authentic 

 specimen of Cyclotella maxima Kg. from Peru {leg. Hayne). 

 Fig. 2 h, structure of the inner part of the valve (^V"^)- Fig. 2 c, 

 structure of the margin (^-n;^'). Fig. d, two frustules cohering by 

 a broad connecting membrane, of rare occurrence, but which 

 evidently shows how nearly allied P. maxima is to P. hormoides. 

 The two frustules exhibit the granular arrangement of the 

 endochrome (^). 



P. liormoides Montague {nee Smith) is nearly allied to 

 P. maxima, and differs only by its smaller size, more convex 

 valves, and somewhat closer puncta. The cell contents are 

 granular. As with other diatoms, we find in some gatherings 

 of P. hormoides and P. maxima certain specimens not varying 

 much in size, which lead to the conclusion that we have to do with 

 distinct species ; but in gatherings from other localities inter- 

 mediate forms appear, and I have scarcely any doubt that P. 

 maxima is only a large form of P. hormoides. PI. XXL, Fig. 7 a 

 (°-^), authentic specimen from Lima. Fig. Ih {^-^^), structure 

 of the valve. 



P. amhigua Grun. in Cleve and Moller, 'Arctic Diatoms.' 

 (Hyalodiscus stelliger Grun. nee Bailey Novara ex parte.) — This 

 species is distinguished from P. maxima by a much larger and 

 more sharply defined dark umbilicus, its smaller size and somewhat 

 closer rows of puncta (15-17 in "01 mm. in the middle of the 

 valve). Common at the island of St. Paul, Cape of Good Hope, 

 Kerguelen's Land, &c., and is very constant in these localities, 

 where P. maxima does not occur. I have seen specimens of 

 P. maxima from other localities with a small umbilicus, and other 

 intermediate forms may probably be detected. The cell contents 

 are granular. Hyalodiscus subtilis is distinguished by its much 

 finer striation and coherent endochrome, which sometimes form 

 radiant lobes easily visible even in dried specimens. A very large 

 form of P. amhigua (var. Kamschatica), attaining a diameter of 



