18 JOHNSTON, ON DIATOMACK^. 



rectly referred to this genus, and is probably anew and certainly 

 a fine species. The absence of any trace of central nodule 

 would seem to exclude it from Professor Smith's definition 

 of the genus, but it is not unfrequently absent in the lower 

 valves of many species. 



Stauroneis angulata, Johnston. — The correct position of 

 this species appears to have been misunderstood by the 

 author, as there is no true stauros, and there can be but little 

 doubt that it is either the Navicula Hennedyii of the ( Synop- 

 sis'* and Professor Gregory's Glenshira sand,f or a variety 

 of that form. It agrees closely in size with the dimensions 

 given by Professor Smith, and the whole structure of the 

 valve is identically the same. 



Heliopelta Phaeton, Johnston. — The true generic posi- 

 tion of this species has also been quite overlooked, as any 

 reference to Ehrenberg's description and figures of Heliopelta 

 would have shown. It ought evidently to be referred to one 

 of the numerous forms of Actinoptychus or Actinocyclus, 

 and is probably Actinoeyclus duodena rim of Smith's ' Synop- 

 sis/ vol. ii, p 86, or the Actinoptychus duodenarius of the 

 ' Mikrog./ t. xviii, f. 24. A description and figure of Helio- 

 pelta will be found in l Silliman's Journal' for 18 15, vol. xlviii, 

 p. 338, tab. vi, fig. r. 



The Arachnoidiscus ? is the A. Ehreiibtrr/u. Sec 



Dr. Walker-Arnott, in 'Micr. Journ./ vol. vi, p. 162. 



Pleurosigma makron, though large, and probably from 

 that cause with more strongly marked strirc, agree so closely, 

 both in form and structure with P. Balticum of Professor 

 Smith, that there can be no grounds for theirs cpara- tion 

 from that species. 



Note on Figs. 13 and 14, by Dr. Walker-Arnott. 



Both these forms clearly belong to the genus Gcphvria, 

 established by me in ( Micr. Journ./ vol. vi, pp. 163-1, and 

 not to Achnanthes. 



I have also stated in ' Micr. Journ./ vol. vi, p. 195, tli.it 

 the Entopyla australis of Ehrenberg (' Berlin Proceed bags/ 

 1818, p. 7) was perhaps the same as my Gephyria incurvata, 

 from Ichaboe, or, at least, partly so. I arrived at this eon- 

 elusion principally from supposing that Ehrenberg's sample 

 of guano was not from Patagonia, as he had been informed, 

 but from Ichaboe. Fig. 10, however, of Mr. Johnston's, 

 taken from specimens from Patagonian guano, so far as I 



* Smith's 'Syn.,' vol. ii, p. 93. 

 f ' Micr. Jour.,' vol. iv, t. v, f. 3. 



