296 Proceedings of the Royal Irish Academy. 
the spiral with the divided scale is attained by means of a triangular 
piece, which slides along the slit, and one edge of which is cut accord- 
ing to the same spiral as that traced on the cylinder or axis. It is 
thus easy to bring this edge to coincide with the portion of the spiral 
showing itself between the two edges of the slit ; a line traced trans- 
versely on this slide-piece, at the base of the triangle, indicates the 
point of coincidence of the spiral with the divided edge, and therefore 
shows the corresponding number of degrees, while the opposite side of 
the slide-piece carries the vernier giving the fraction of degree. 
I have not introduced any tangential screw, as practically un- 
necessary, and tending to complicate the instrument ; nor have I made 
any special arrangement for setting up the instrument in a fixed posi- 
tion, on a stand, as I would prefer leaving that to the skill, and prac- 
tical ingenuity of the student ; it being precisely one of those simple 
mechanical problems which chemists have frequently to solve, and 
which may be solved in many simple and sufficient ways. The essen- 
tial portions of the instrument existing, any further addition must 
increase its bulk, render it more costly, and in no way add to its 
accuracy. 
DESCEIPTIOi^r OF PLATE XX. 
a. Crystal, 
b. "Wax attaching crystal to moveable cap C. C, fixed upon the inner revolving 
axis. 
c. d. Screw Cup retaining the outer revolving axis in cylindrical cover. 
d. d. Runner carrying the vernier k. k. 
/. Head by which the outer revolving axis is turned by hand, so as to give 
measurement. 
ff. Head of inner revolving axis, allowing of proper disposal of crystal for 
measurement. 
k. Vernier. 
h. h. Slit in cylindrical cover, and having the opposite edges divided. 
m. m. Spiral traced on outer revolving axis, as seen in contact with edges of 
slit. 
XXXIY. — Notice op the Genus Tetrapedia (Eeinsch), and op tavo 
KINDRED NEW FoRMS. By WlLLIAM ArCHER. 
[Eead June 24, 1872,] 
Amongst ''Unicellular" Algae (as well in the wider as in the most 
strict sense), falling under the class Chlorophyllaceae, forms with spe- 
cially figured cells — that is, otherwise than globular, ellipsoidal, or 
cylindrical, with more or less abruptly or broadly rounded ends— are, 
as is well known, numerous. One has only to call to mind, not to 
