FRESHWATER ALGM 281 



possessed a similar transversely expanded, asymmetrical central area, and the lunate 

 terminal areas. The latter feature is also possessed by C. pellucida Hantzsch (cf. 

 A. Schmidt's Atlas Dial. t. 195, f. 1-6.) 



NAVICULACE.E 



Genus TROPIDONEIS Cle_ve. 



27 Tropidoneis Icevissima sp. n. (PL XXVI, Figs. 115-120) 



T. parva, delicatissima et laevissima; valvis oblongo-linearis, diametro 8-11-plo 

 longioribus, in parte mediana marginibus parallelibus, apices versus leviter et gradatim 

 angustioribus, polis obtuse rotunda tis, raphe recta sed juxta polos levissime curvata 

 nodulo centrali in staurum transversum producto, stauro angustissimo plerumqu 

 valvae margines versus paullulo dilatato, alis (vel carinis) carentibus, striis non visis 

 Cellula in aspectu cingulato anguste oblongo-rectangulari, medio constricta, lateribu? 

 convexis et angulis rotundatis. 



Long. valv. 49-98 M ; lat. med. 8-9.5 /*. 



Hob. Clear Lake, Green Lake, and lake on west side of McMurdo Sound. 



This diatom was one of the most abundant species in the Antarctic collections 

 occurring in great quantity in several of the lakes, both freshwater and strongly saline 



It belongs to the section Orthotropis of the genus Tropidoneis, having a raphe 

 which is practically straight. The valves are very convex (consult Fig. 120), with a 

 slight thickening along the central axis, but they are entirely destitute of wings (or 

 keels). The great convexity of the valves, the slight indication of the bending of the 

 raphe near the poles, and the form of the girdle- view are all characters which place 

 the diatom in the genus Tropidoneis. The markings, if any are present, must be 

 exceedingly fine. Carefully prepared valves were examined most minutely with a 

 Leitz oil-immersion lens giving a magnification of 2000 diameters, but no trace of 

 striations could be discovered. 



The central stauros is very narrow and reaches to the edges of the valve, where it 

 is slightly dilated. The general character of this stauros is very similar to that of 

 the pelagic Navicula (Stauroneis) Biblos Cleve (Le Diatomiste, Mar. 1892, p. 77, t. 12, 

 f. 9, 10), but the other characters of the diatom are very different. A stauros of a 

 similar kind is found in several species of Stauroneis, and there is a slight resemblance 

 between Tropidoneis Iwvissima and Stauroneis spicula Dickie (vide Van Heurck, 

 Synops. Diat. Belg. t. 4, f. 9), but the valves are not so attenuated and the striations 

 (if present) are not nearly so strong. 



