192 Rhodora [OCTOBER 
without serious interruption. The isolation of the island, lack of 
inflowing streams, etc., very greatly limit the chance of importation 
of new diatom species to these ponds. 
The map shows the locations where diatom samples were secured. 
All of the ponds are very small and shallow. The samples were 
dredged at a depth of about one foot, except in the cases of the marsh 
and Dry Pond, both of which had dried to a condition of cracked 
mud, and here some of the surface was collected. The diatom flora 
of any of the ponds is, as intimated, rather meager, and in several 
of them the conditions seem to have been favorable to the rapid 
multiplication and growth of a certain single species. In Typha Pond 
Navicula elegans W. S. is very abundant, in Tub Ponds Navicula 
peregrina E., and in South Pond Navicula formosa Greg. is dominant. 
The finding of a mixture of distinctly fresh- and salt-water forms 
in several of the ponds is probably not difficult to understand. Dr. 
H. W. Henshaw has suggested that the isolation of the island and the 
small amount of fresh water on it places the latter at a high premium 
with the terns and other aquatic birds which make their home and 
nest there by the thousands. Hence it is easy to see how marine 
diatoms would be carried by them to the fresh-water ponds where 
they must necessarily go daily by thousands to drink. Overflow 
by the sea in times of high tide or high winds would also help to 
explain the presence of marine diatoms in those ponds of lower 
elevation and in the marsh. | 
Two species of Navicula have proved indeterminate thus far, and 
the following fresh-water form is new. 
Navicula nanella P. C. sp. n. Valve elliptical with broad obtuse 
ends and straight taper from the center to the ends. Lunate thicken- 
ing across extreme ends of valve from the terminal nodule to the 
margin. Central area broadened laterally. Arrangement of costae 
giving appearance of a double dark band across the valve near center 
when dry. Raphe straight, slightly swelling at the center. Costae 
widely spaced, heavy, smooth, and reaching nearly to the raphe, 
radiate from the center half way to the end of the valve, and conver- 
gent from thence on to the end. Length 0.023 mm. Width 0.008 
mm. Striae 8% in 0.01 mm. 
Fresh water. 
This species presents some likeness to Navicula hungurica Grun. 
(1860), see Cleve, Nav. Diat. II, p. 16, but if so, Cleve's interpretation 
of the strongly marked costae on either side of the terminal nodule 
must refer rather to an internal thickening of the valve at this point, 
