216 PROCEEDINGS OF THE 



This occurred in a sample I received from the late Dr Gregory. 

 It is from Elgin, and distributed by him as E. 15; of this I have 

 given slides to many friends as my number 156, so that those who 

 possess such may perhaps find more; but a very careful search is 

 requisite. The valve is oval, not unlike that of Nav. ajnculata 

 (Nav. placentula of Lewis, but not of Ehrenberg), but has not the 

 apiculi at the ends. 



Cocconeis Thivaitesii, Sm., is now by most referred to Achnan- 

 thidium {A.flexellum,, Breb.), but the general aspect is very unlike 

 any other of that genus. In this species the upper valve has a 

 median line, but no true nodule; in place of this last, there is a 

 hazy expansion apparently within tlie substance of the silex. This 

 is sometimes small, but often assumes a nearly circular or square 

 appearance, extending almost half-way to the margin. "W^hen this 

 is seen, we have the Navicula, macula of Gregory, described by him 

 from the Glenshira sand; indeed, Gregory says, of that supposed 

 species, that it is not unlike the larger specimens ef Cocc. Thivaitesii. 

 It is best seen in deposits (as from Watten Lake, Caithness); 

 but in all recent samples which I have examined I have found 

 traces of it. When the two valves are still united, both the hazy 

 expansion and the stauros may be seen by slightly altering the 

 focus. Another variety of C. Thivaitesii (or Achn. flexelkim) is 

 linear oblong, with nearly parallel sides. This was described by 

 Professor Smith in his paper on Pyrenean Diatoms, under the 

 name of Navicula dissimilis. It has been found on Ben Lawers, 

 near Loch Tummell, banks of Loch Leven, near Kinross, and 

 about Hull, but is usually scarce in each locality. In it, also, I 

 have observed the hazy expansion on the upper valve. 



II. — NiTZSCHIA. 



This genus was proposed by Mr Hassall for what he calls JV. 

 elongata, now known by the name of N. sigmoidea,"' but the defini- 

 tion he gives is of no value. It was adopted by Mr Smith, but 

 not sufficiently limited. The valves, according to him, are keeled 

 (the keel frequently eccentric) with one or more longitudinal lines 



* N. sigmoidea B of Smith, is quite distinct from that species, and agrees ^vith 

 Synedra spectabilis Ehr., which, however, is a true species of Nitzschia (N. specta- 

 lilis, E,alfs in Pritch. Inf., p. 782, but not N. spectabilis of Smith). Sigmatella. 

 Brehissonii Kiitz. is not N. Brcbissonii Sm., as he supposed, but a slight variety, 

 N. sigmoidea having the same difficult striation. 



