in Sand from Sahara Desert. ` 403 
paper, all the forms are represented which appeared specifi- 
cally distinct. It is, however, very possible, especially as my 
acquaintance with the genera and species of the polythalamia 
is but slight, that other species may have escaped notice 
among the large number of worn and broken specimens. 
The figures given were sketched by means of a camera 
lucida eye-piece attached to one of Chevalier's Horizonto- 
vertical microscopes. They are all drawn to one scale, indi- 
cated by fig. 10, (Pl. XX.) which represents 3% of a millimetre, 
magnified equally with the sketches. By means of this scale, 
the dimensions of the specimens figured can be readily ascer- 
tained, 
I have determined two only among the species figured. 
The one represented in figs. 1 and 2 is the Textilaria globu- 
losa: fig. 9 is the Rotalia globulosa. Figs. 3 and 4 I have 
no doubt belong to the genus Textilaria ; probably fig. 4 is 
the T. aciculata. "The cells of these polythalamia are often 
partially or entirely filled with opake calcareous matter. More 
generally, the cells are empty ; or only dark spots, such as are 
represented in figs. 2, 7, and 9, are seen in them. The aver- 
age size of the shells is about the same as that of the quartz 
grains. But there are many fragments present which must 
have belonged to larger individuals than any that were found 
entire. No siliceous infusoria were met with. The imper- 
fect state of the specimens prevented my giving outlines of 
them in other positions than those in which they are figured. 
For the same reason, it was not practicable to estimate accu- 
tately the proportion of individuals belonging to the several 
Species represented, but the Textilaria globulosa and the form 
"Presented in figs. 6 and 7, are the predominant species. 
Only a few specimens of Rotalia globulosa were found. 
I am unable to draw any conclusion with regard to the 
Beologica] age of the Desert Sand from the presence of these 
Poly thalamia, The species Textilaria globulosa, and T. acicu- 
fa; and Rotalia globulosa are among the most prevalent forms 
9f the chalk formation ; occurring in the cretaceous strata of 
