NATURAL HISTORY SOCIETY OF GLASGOW. 211 



The Plate accompanying this notice is from the pencil of Mr 

 William Sinclair; the upper figure is that of the bird exhibited, 

 the lower one represents a male from the cabinet of Mr E. E. 

 Alston of Glasgow. 



II. — Notes on Cocconeis, NitzscMa, and some of the allied genera of 

 Diatomaceoe. By Professor Gr. A. Walker-Arnott. 



I. — Cocconeis. 



In Smith's synopsis of British Diatomacece, the only systematic 

 work on the subject of any value, this genus would appear only 

 to differ from Navicula by the frustules being adherent, or 

 attached by the surface of the lower valve to larger algce. Smith 

 adds, that "the markings of the lower are less intense than those 

 of the upper surface, but in other respects the valves are sym- 

 metrical, and Mr West informs me that he has found the absence 

 of a central nodule in the lower valve, as noticed by some writers, 

 by no means a constant character." This observation is correct as 

 to the marking of the lower valve, but there is a double error in 

 regard to its nodule. 



No author known to me mentions that the central nodule of the 

 lower valve is absent; and it is therefore probable that West meant 

 to speak of its absence from the ^qoper valve. Kiitzing, in liis 

 Bacillarien, p. 70, places Cocconeis among his Monostomaticce "ostiolo 

 medio in latere secundario inferiori, in superiori nullo," or, to use 

 Smith's terms (now generally adopted, for there is no opening), "a 

 central nodule in the lower valve, none in the upjjer." The same 

 structure is indicated in his Species Algarum, at p. 50, and there 

 seems to be no exception to this character in well-known species : 

 when such occur, the species exhibiting it will probably be found 

 to be generically distinct. The upper valve has usually no mark- 

 ings at the place where the nodule and median line might be 

 expected, and when the two valves are united to form the entire 

 frustule, we see the nodule and median line of the lower valve 

 through the upper one, leading one to suppose that what was 

 before us was the upper valve with a median line and central 

 nodule. As neither Smith nor any other author has given figures 

 of both valves of the several species, our information regarding the 

 species is as yet very imperfect. No figures can afford good 

 illustrations unless the specimens be obtained from the Aveed on 

 which the species gTows, and then at least three are required : one 



