Barber. — On the Structure of P achy theca. 147 
peripheral band, and the crystalline centre has a diameter 
fully three-fourths of that of the whole sphere. There is not 
usually any structure to be made out in the central part ; but 
in one case it appears to have distributed through its substance 
numerous dark dots. Finally, in one specimen, whose section 
appears on the surface of the stone, there appears to be a 
second dark ring at some distance from the cortex, and com- 
pletely surrounding it. 
It may be noted that the only specimens showing the 
microscopical structure described in this paper were found in 
the Wenlock Limestone : and the only reasons we have for 
regarding these bodies as identical with such as are frequently 
met with in the Transition Beds, are their spherical shape, 
and the general arrangement of the filaments composing the 
cortex. The Pachytheca of the Bone-bed and the Downton 
Sandstone appears to differ from that of the Wenlock Lime- 
stone in certain particulars ; and a comparison of the drawings 
made from specimens obtained in the sandstones, illuminated 
by reflected light, with the transparent sections from the 
Wenlock Limestone, suggests that we may have to do with 
two entirely different organisms l . In no specimens from the 
Downton Sandstone have I found any trace of the circular 
spaces at the base of the radiating filaments, which form so 
puzzling a feature in the sections from the Wenlock Limestone 
possessed by Sir Joseph Hooker. As far as can be judged 
from the sandstone specimens, the radiating dark lines do 
not swell out much at their inner ends, nor do they embrace 
circular spaces (Figs. 11, 11 a, 11 b). 
1 Since writing this paper I have had the opportunity of examining a section of 
Pachytheca from Corwen, which is preserved in the Jermyn Street Museum. Al- 
though the structure is not very well shown, a comparison certainly strengthens the 
suggestion that several different organisms have been included under the name of 
Pachytheca. 
