458 PROCEEDINGS OF THE ACADEMY OF [May, 



Fresh water. Fossil and recent. 



Fall Mountain, Conn. W. A. Terry, 1890. 



This diatom presents several curious features. It is the largest by 

 far of the genus Stauroneis. It is the only one of the genus to show 

 subterminal depressions on the valves. It is the only naviculoid 

 diatom known to me that has a silicious belt about its middle part. 

 For these and other reasons, it is of the greatest interest. The anom- 

 alous features specified might easily lead to the suspicion that we 

 have here a monstrous form, a "sport," an abnormal condition of the 

 species with which it is associated, Stauroneis acuta W. Smith. This 

 idea might seem to find some confirmation in the fact that the striation 

 and punctation, and the shape of the lower valve are approximately 

 those of that species. 



But monstrous forms do not, usually if ever, continue to produce 

 themselves indefinitely, so that one shall find them in both the mud 

 beneath and the water above. Monstrosities do not, like this form, 

 recur not only for years and years in one place, but also in other 

 ponds and streams of the same region. 4 Monstrosities are not, like 

 this diatom, fairly uniform in characters, so that one picks out hundreds 

 of quite similar specimens from the material. Whatever may be the 

 relationship. Mr. Terry's diatom is something more than a teratological 

 condition of St. acuta. 



Some possibilities suggest themselves. Unquestionably the rela- 

 tionship is close. The peculiar silicious thickening at the ends of the 

 lower valve is precisely alike in both, and this is a very significant fact. 

 The striation and the beading are essentially similar. The lower 

 valve, apart from its size, might upon superficial study be thought 

 identical with the valves of St. acuta. The outline of this diatom, 

 however, is different, even in the lower valve. Also, the walls of both 

 valves are thinner, and the whole frustule is more nearly circular 

 in cross-section. The size is relatively colossal, being more than 

 twice as long as the ordinary St. acuta. There are no girdle-bands in 

 any of the hundreds of frustules examined or mounted, nor were 

 any bands found in the accompanying material that could possibly 

 belong to this form. No instance of reduplication has been observed, 

 and as a consequence, the ultimate fate of the unique silicious ring or 

 belt is not known, though one must think its rupture a condition 

 precedent to this process. 



Some of these features might seem to point toward the conclusion 



* Mr. Terry finds it in "three widely separated ponds in Bristol." Rhodora, 

 August, 1907. 



