16 JOHNSTON, ON DIATOM A CE<E. 



frustule towards either end j and, 3d, the disproportion 

 between the two diameters — 19 to 1 ; features sufficient to 

 establish a variety, but not, as was remarked by A. M. 

 Edwards, Esq., of such importance as to justify the erection 

 of a species. 



It may be stated that Pongateague Creek, the habitat of 

 these large diatoms, is a shallow inlet of the Chesapeake 

 visited by the tide. 



Baltimore, Maryland; 

 May 20th, 1859. 



We print the foregoing communication as containing de- 

 scriptions of Diatomacese from a locality which has hitherto 

 been little noticed, but as the author appears to have over- 

 looked the works of some writers in this country, we append 

 the following notes on some of the species by Professor 

 Walker- Arnott and Mr. F. C. S. Roper. 



Note by Mr. Roper. 



The species described by Mr. Johnston in the foregoing 

 paper appear, as far as I can make out from his descriptions 

 and the very well-drawn figures that accompany them, to be 

 mostly known to observers in this country. The fact that lie 

 notices with respect to Spatangidum Ralfsianum, Grev., of the 

 rays in the two valves being disposed alternately, so that the 

 ray on the superior valve is opposite" to the areolated portion 

 of the lower in the perfect frustule, has already been noticed 

 by Mr. Shadbolt,* in Asterolampra, a kindred genus, and, I 

 believe, is well known to be the constant rule in both Spa- 

 tangidum and Asteromphalus, should these genera, on more 

 careful consideration, prove distinct. They have been 

 adopted, however, by such acute observers as Dr. Greville 

 and M. De Brebisson, and though they appear to differ as to 

 the limits of each, my own impression is that both are formed 

 on one common plan, and should both be united under 

 Ehrcnberg's original genus of Asteromphalus, the differences 

 only being sufficient to afford specific characters. 



Asteromphalus centraster, Johnston (tig. 10). — It is impos- 

 sible to form an opinion on this species without seeing the 

 specimens. The structure, both from the description and 

 figure, appears to differ materially both from the genus 

 * ' Micr. Jour.,' vol. ii, i>. 17. 



