20 Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 
The genus Solenopora was originally described by Dr. Dybowskt 
(Cheetetiden der Ost-Baltischen Silur-Formation, p. 124, 1877) for 
the reception of a singular fossil which is of common occurrence 
in the Ordovician limestones of certain localities in Esthonia.. The 
structure of this organism has been fully dealt with by Mr. R. 
Etheridge, jun., and myself (Gron. Mac. Dec. III. Vol. II. p. 529, 
PL. XIIL.), and we have shown its identity with the forms previously 
described by Mr. Billings as Stromatopora compacta, and by our- 
selves as Tetradium Peachii. The species stands, therefore, now as 
Solenopora compacta, Billings, the Scotch examples remaining as a 
variety, for which the name Peachii may be employed. 
B 
Fic. 8.—A, A small specimen of Solenopora compacta, Billings, from Saak, Esthonia, 
of the natural size. B, Surface of a piece ot limestone largely made up of small 
specimens of the same, from the same locality, of the natural size. C, Tangential 
section of the same, enlarged about 36 times. JD, Vertical section of the same, 
similarly enlarged. 
SoLENopoRA compacta, Billings, sp. Fig. 3. 
This species has been so fully treated previously (loc. cit. supra), 
that it is unnecessary for me to enter here into any discussion of its 
characters. The accompanying illustration (Fig. 3) will sufficiently 
indicate its general form, mode of occurrence, and minute structure. 
As a species it is distinguished by the size of its component tubes 
(which vary in diameter from ;’y to ‘5 of a millimetre), and by the 
facts that the tubes are irregular in shape, have undulated walls, and 
are furnished with more or less numerous septiform processes due to 
fission (Fig. 8, C and D). 
It has been already shown by Mr. R. Etheridge, jun., and myself 
(loc. cit.), that Solenopora compacta, Billings, is very widely dis- 
tributed in the Ordovician rocks, and that it played a very important 
part in the formation of certain of the limestones of this period. 
