70 
GEOLOGY OF THE FIRST DISTRICT. 
on the backs of watches ; at other times the spots are found to form three sets of lines, making 
angles of 60° and 120° with each other, as shown in Fig. 12 ; and on others the spots are dis¬ 
posed without much apparent regularity, frequently having a star-like figure in the centre. 
The spots are so small on some of the discs, as to 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. October \Qtli, 1841. Since the above was ready for the press, I have seen, in the ap¬ 
pendix to Pritchard’s History of Infusoria living and fossil, some interesting statements of recent 
discoveries by Ehrenberg, with reference to the genera of Actinocyclus and Coscinodiscus. 
It appears that these genera, which were first discovered in a fossil state in the schiste of Oran, 
Caltasinetta, 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 
senarius, Coscinodiscus patina, and Gaillonella sulcata, species now living, may be shown 
as the chief forms met with in the chalk marls of Sicily, and also that the species of the chalk 
formations are yet to be found as crowds of living creatures in the waters of our seas. 
I select from the species of Coscinodiscus, described by Ehrenberg, the following, as appa¬ 
rently identical with American species from Richmond, Va. In connection with the descrip¬ 
tion, I give a reference to figures drawn by me from fossil American species, long before 
Ehrenberg’s characters for the species were received. 
Coscinodiscus lineatus. (PL 2, fig. 12, a. b.) Carapace marked by small cells disposed in a 
series of parallel and transverse lines. Found fossil in the chalk marl of Caltasinetta, 
and in the live condition at Cuxhaven. The cells in this species form parallel lines in 
whatever direction they may be viewed. In large and well preserved fossil specimens, 
as many as twenty-five openings were seen near the circumference. Within the live 
forms, numerous yellow vesicles are sometimes seen, as in Gaillonella. Diameter of 
fossil, yy’joth to jljth; living yJ^pth to ^^^th. Fossil at Richmond, Va. 
Coscinodiscus radiatus. (PL 22, fig. 14.) Carapace large, marked with cells of moderate 
size, disposed in lines radiating from the centre. Towards the margin, the cells become 
smaller in size. Very abundant in fossil state at Oran; alive near Wismar and Cux¬ 
haven ; to gy ptli- Fossil at Richmond, Va. 
Coscinodiscus argus. (? Var. of C. radiatus.) Carapace with large cells at the centre, and 
smaller ones at the circumference, the order of the rays being often interrupted. Fossil 
at Oran and Caltasinetta in chalk marl, living in sea water at Cuxhaven. The cells of 
the discs from Oran vary very much in size. The ova are of a greenish color in the 
living forms, which are very rare. Diam. p^pth to aipth. Fossil at Richmond, Va. 
Coscinodiscus oculus-iridis. Carapace marked with rather large radiant cells, except near the 
centre and circumference, where they are smaller. Some of the larger cells in the centre 
form a sort of star. Fossil in the chalk marl of Greece ; alive near Cuxhaven. Dia¬ 
meter, 2 I 0 . This large species is curiously marked, whilst under the microscope, with 
