J 
Navicula ————(?), z35 (8:7:8). s10, v37 (8:7:8) s9. May be a new species not 
distant from JN. borealis. 
Nitzschia amphibia 17 (1:4:1) s8 and 16. Not rare. 
a vermicularis (KK) Grun. 100 (43) s8 and ?.. Not common. 
Y a (2?) 103 (10:15:13:15:10) s43 and 18. Not rare. May be 
new. 
i -—_————(?) 32 (5) s7 and 15. Rare. 
Stauronets phoenicenteron E. Rare. 
Synedra ulna E. Rare. 
The most interesting feature of this collection is that it affords positive proof 
that two Diatoms previously referred not only to distinct species but also to 
two distinct genera are‘in reality but two different plates: of a single species. 
The two forms thus regarded as being even generically different are Navicula 
cuspidata KK. and Surtrella craticula—both of similar shape and dimensions, but 
with the sculpture of the former consisting only of fine striae, closely approximated 
and rectangular to the raphe, while the second exhibits a surface which is strongly 
and irregularly craticulated. In the Airdrie collection both of these are found 
rather abundantly, but with them are others less abundant, in which the features 
thus referred to are both found in the same specimens, difference of focussing 
being all that is required to bring the one or the other into view, as may be 
desired. By the same focussing the craticular form of Surirella craticula is 
proved to be the inner plate of Navicula cuspidata. 
Dr. Mackay, on the receipt of a mounted sample of this collection from 
Mr. O. Kendall of Providence, R.I., U.S.A., accompanied by photomicrographs 
herewith reproduced, maintains that a diatom of the Navicula cuspidata group, 
which abounds in a conspicuously fine craticulated form, is of co-ordinate specific 
or varietal value to ambigua E., halophila Grun. and its larger variety major 
Heribaud, and to Perrotetttt Grun. of Senegal, which it most nearly approaches. 
(See Schmidt’s Atlas, 221:33). Instead of adding to the number of species he 
takes at present the more conservative course of treating all these forms as 
varieties of the species cuspidaia. On the other hand he is equally ready to 
allow them as species of the N. cuspidata group. Ambigua, in both the normal 
and craticulated form, is sharply distinguished from the others by its rostro- 
capitate ends, and the others by the rounding out of the rostrate ends of the 
type cuspidata. 
These three other forms are of a similar type. Tapering from more or less 
narrow but rounded ends they swell more or less evenly to a maximum breadth 
at the centre, the exact dimensions of which can be comparatively shown in 
Dr. MacKay’s notation as follows: * 
IN. halophata Grun. 50 ( -:10: .).s19 to 70 (- -:12: +.) s20. 
N. halophila V. major Her. 110 (4:17:4) s16 to 136 (4:20:4). 
N. Perrotettit Grun. 175 (16:41:16), Schmidt’s Atlas, 211:33. 
*The numbers are microns—the length standing first, the breadths at the principal points 
of flexure along the margin of the valve being within the parenthesis separated by colons. 
The letter “S’” following stands for “‘sculpture’’ and the number following for the number of 
lines or dot elements in the sculpture in the space of ten microns. 
163 
