Diatoms from Chiucha Guaun. -185 



only in the groatcr relative lengtli of the uwiis and the size, 

 which seems to be larger ; tlie surface of the endocysts is also 

 granuhited. It might represent only a variety of G. incurvut. 

 I give here some measurements of the latter: — 



General length : 00i>3 028 0029 0027 X 



Lengrth of the valve: 0-014 OOH) OOU) OOlUo 0021- 

 Brt'udtli of tlie valve: "x" O^OlU 010 OUOtJ OWih 



10. Chaetoceros Lorenzianus, CJnin. Very common. 



17. Chaetoceros peruvianus, Brightw. Rare. 



There are a few more species of this genus, which, however, 

 I was unable to determine. 



18. Rhizosolenia styliformis, Brightw. Very common. 



19. Skeletonema costatnm (Grev.), CI. Very common. 



20. Skeletonema costatum, var. spiralis, var. nov. (PI. XVf. 



tig. 8.) Not rare. 



Differs from the type only in the oblique disposition of the 

 spines, which unite the frustules together; the degree of in- 

 clination of the jipines is variable, and there are always some 

 frustules in the same filament whose spines are straight, as in 

 the type species. 



Although Skeletonema costatum is known to me from very 

 different localities all over the world, I have never before 

 seen such a spiral di.-position of the spines ; it seems therefore 

 advisable to regard the present form as a separate variety. 



21. Eucampia zodiacus, Ehr. Rather rare. 



22. Biddulphia (Triceratium) alternanSj Bailey. Very 



common. 



2.'j. Biddulphia (Triceratium) alternans, var. variabilis, 

 Brightw. Rare. 



These two forms are generally regarded as belonging to 

 two distinct species — the second being cliaracterized by the 

 irregular form of the valve, its concave margins and the acute 

 apices of the three ends; the first having a regular form, 

 straight margins, and broadly rounded apices. The exam- 

 ination of both of these sjecies from Chincha guano, in which 



