OF THE FAMILY BACILLARIA. 141 



same species also occur fossil in cliffs on the Rappahannock River. 

 In figs. 12, 13, and 14, are represented the discs which I believe 

 to belong to the genus Coscinodiscus. When perfect, the form 

 seems to be that of a torus, having the circular bases covered with 

 hexagonal or circular spots, which present considerable variety 

 in their size and arrangement in different specimens. The most 

 usual disposition of the spots is in rows coiTesponding with the 

 radii, as shown in the large specimen^ fig. 14. In consequence of 

 this arrangement, they also form beautiful spiral rows in other 

 directions, so that the curves present no inconsiderable resemblance 

 to those often seen on the back of watches ; at other times the 

 spots are found to form three sets of lines, making angles of sixty 

 degrees and one hundred and twenty degrees with each other, as 

 shown in fig. 12, and on others the spots are disposed without 

 much apparent regularity, frequently having a star-like figure in 

 the centre. The spots are so small on some of the discs, as t(?be 

 almost invisible even by the highest magnifying powers ; on 

 others, as in fig. 14, they are quite large and distinctly hexagonal. 

 The largest discs have not always the largest spots. There are 

 certainly several species of this genus in the infusorial stratum of 

 Richmond, Va., but as I have not seen Ehrenberg's account of 

 the European species, I cannot venture to name our own. 



Note, Oct. lO^A, 1841. — Since the above was ready for the 

 press, I have seen in the appendix to Pritchard's History of Infu- 

 soria, living and fossil, some interesting statements of recent 

 discoveries by Ehrenberg, with reference to the genera of Acti- 

 nocyclus and Coscinodiscus. It appears that these genera, which 

 were first discovered in a fossil state in the schists of Oran, Cal- 

 tasinetta, Zante, &c., have also been recently found in sea water,* 

 and that many of the living species are identical with the fossil 

 ones ; indeed, Ehrenberg states that Actinocyclus senariKs, Cos- 

 cinodiscus patina, and Gaillonella sulcata, species now living, 



* Since the above note was written, I have found several species of Coscinodiscus and 

 Actniocychis, in the Uving- state, quite abundant in the harbors of New York and Boston, 

 and also in the Hudson River at West Point. 



