REPORT OX THE DIATOMACE.E 51 



The frustules of this type, which was collected in the Antarctic Ocean, show a charac- 

 teristic peculiarity at their inferior extremities, and one which is not found in any frustule 

 of Asterionella bleakeleyii, W. Sm., namely, the presence, on the zonal side, of two small 

 symmetrical lines, which run in the direction of their long axes. These probably represent 

 the incapsuling of the two valves of the frustule, and if so, ought to have been visible 

 in Asterionella bleakeleyii, though contracted as a result of incineration. 



Nor can Asterionella glacialis be confounded with the freshwater Asterionella 

 Jbrmosa of Hassall, 1 as the latter has its superior extremity somewhat swollen, although 

 to a less extent than its inferior extremity, while the former has a swelling only at the 

 inferior extremity of the valve, and on the zonal side of the frustule, so that it must be 

 regarded as a new species. 



Asterionella gracillima (Hantzsch.), Heib. (Plate XXV. fig. G.) 



We have here represented a form which cannot be confounded with ^the Asterionella 

 bleakeleyii of W. Smith, as, instead of being linear in the parts succeeding the dilated base, it 

 is sensibly swollen at its superior extremity. The specimen, however, which unfortunately 

 has suffered somewhat from the bleaching process, agrees better with Asterionella gracillima" 

 (Hantzsch.), Heib., in its morphological characteristics ; but, instead of being found in fresh 

 water, like the latter, it occurred on the surface of the sea. As it may have been borne 

 to that locality by glaciers and icebergs, it may, for the present, be viewed as identical 

 with that species. 



Synedra, Ehrenb. 



This genus is chiefly characterised by having its small frustules joined by means of a 

 cushion, or sometimes of a more or less developed peduncle to seaweeds or other objects. 

 The attachment thus effected is of a feeble kind, so that it is a very common occurrence 

 to find living specimens in a free state, and in the examination of geological diatomaceous 

 deposits, or of frustules that have been treated with acids, it is impossible to determine 

 whether any given form existed in the free or adherent condition. But apart from its 

 attached or non-attached habit, the genus cannot, on account of its structural peculiarities, 

 be confounded with any other. Its closest affinity is to be found in the Bacillarice, from 

 which, however, it is readily distinguishable by the presence in the latter of a characteristic 

 punctated keel. Nor can it be confounded with the Asterionella, Hass. , or with members 

 of the genus Thalassiothrix (Grun.), Cstr., inasmuch as it does not possess large broad or 

 dilated extremities, but is provided either with a small cushion, which unites the frustules 

 in a star-like manner, or with small triangular pads placed laterally at the two extremities, 



1 Miar. Journ., vol. viii. pi. vii. fig. 8; Heiberg, Conspec, pi. vi. fig. 20. 



2 Heiberg, Conspec, p. 68, pi. vi. fig. 19 ; -Diatoma gracillimwn, Hantzsch. in Rabenh. Alg., Xo. 1104, c. 

 icone, Krypt. Flor. von Sachsen, p. 32. 



