DIATOMA. 409 



Ralfs. Aberdeen : Dr. Dickie. Stevenston, Ayrshire : 

 Rev. D. Landsborough. 



This is a very variable species. .In the young state the 

 frustules are plain, but in their fully developed condition 

 they are dilated laterally, or as most writers would say, at 

 each extremity. These two states have been separately de- 

 scribed as distinct species, but I have no hesitation in uniting 

 them. I have preserved the specific appellation of elonga- 

 tum in preference to that of tenue, because the latter term was 

 applied by Ehrenberg to the condition of the species with 

 dilated frustules, in fact to the species in its fully developed 

 or perfect state. 



The younger frustules in the end view are slightly dilated 

 in the centre with incrassated ends ; in this state they bear a 

 slight resemblance to the older ones of Diatoma vulgare, but 

 are much smaller. The fully developed frustules in the end 

 view are linear and very slightly incrassated. 



Remarkable as the variety with cuneate frustules is, it is 

 I suspect but an anomalous condition of the species. 



Light brown or yellow when recent, assuming a faint 

 green tinge in drying. 



3. DIATOMA VIRESCENS Hass. 

 Plate XCV. Figs. 7, 8. 



Char. Frustules usually two or three times broader than long, 

 frequently cohering together in the manner of a Fragi- 

 laria. In end view oval, with slightly constricted extre- 

 mities. Strise inconspicuous. 



Fragilaria pectinalis Ehr., Die Infus. p. 206. t. 16. f. 1. 

 excel, a; Pritch. Infus. p. 222. fig. 176. Fragilaria 

 virescens Ralfs, in 1. c. p. 110. pi. 11. fig. 6. ; Jenner, 

 1. c. p. 202. 



Hob. Cold bath spring, Broadwater Forest ; Lower 

 Green ; Rotherfield and Piltdown Common : Mr. Jen- 

 ner. Madron and Chyanhal Moor, near Penzance : 



