146 Barber. — On the Structure of P achy thee a. 
unable to detect any structure in the sections by transmitted 
light. 
I was unable to visit the Wenlock Limestone near Malvern, 
in which Mr. Grindrod found his specimens 1 ; nor was I able 
to visit Corwen quarries, where Hicks found silicified specimens 
of P achy theca and N ematophycus showing structure. Mr. Car- 
ruthers, of the British Museum, tells me that Pachytheca has 
also been obtained from the passage beds at Cardiff, and 
that sections have been prepared of specimens, which exhibit 
structure. 
In the best specimens I obtained it is perfectly easy with 
the naked eye to detect the radiating character of the 
elements composing the wall or cortex of the organism (Figs. 
9 and 10). In such specimens the cortex is black in colour, 
while the central part is filled with white crystalline powder. 
As was pointed out to me by Mr. La Touche, there is 
in most specimens a depression on one side of the rounded 
body (Figs. 8 and 8 a). Mr. Phillips, of Shrewsbury, has sug- 
gested that this depression might indicate the point at which 
the plant was attached to foreign bodies, much as Rivularia 
is now found adhering to sticks. The depression might be 
due to the collapsing of the wall of the hollow organism ; 
this could only occur before the walls assumed the resistant 
nature which has rendered preservation possible. There does 
not appear to be any discontinuity of the filaments at this point. 
In several cases I found small Pachytheca - like bodies ar- 
ranged in rows of four or five (Fig. 14). The occurrence of 
such a condition was not, however, common enough for any 
general conclusion to be drawn from it. 
The centres of the pyritised specimens are usually of a 
crystalline nature. The diameter of this portion, however, as 
compared with that of the whole organism, varies very con- 
siderably. In some specimens the cortical portion is of such 
extent that no space is left in the centre for the crystalline 
matter. In others again the dark cortex appears as a thin 
1 Three days careful examination of the West Malvern Wenlock Limestone in 
January 1889 produced no further specimens of Pachytheca. 
