Barber . — • The Structure of P achy theca, II. 159 
SUMMARY OF RESULTS. 
1. General Structure. P achy theca is a spherical alga con- 
sisting of a mass of cellular filaments. The cells of these 
filaments appear to resemble, in general shape, those of a 
living Cladophora. 
The body of the plant is divisible into a medullary portion, 
in which the cell-rows pass in every direction ; and a cortical 
portion, where the cell-rows are accurately radial, and at 
equal distances from one another. The cells of these two 
regions are continuous with one another ; and the point of 
junction, formerly known as the zone of oval bodies , is charac- 
terised by a change in the mode of petrifaction — probably 
partly dependent upon the alteration in the arrangement of 
the tubes at this point. 
The cortex may be divided into three zones : — an inner zone, 
in which the cell-rows are usually separated widely by lacunae 
in the infiltrating mineral ; a middle zone, consisting of radi- 
ating, for the most part unbranched filaments ; and an outer 
zone, near or at the periphery, where branching is frequent. 
2. The zone of oval bodies (B 1, 2, &c.), the ■ cellular’ 
structure seen in Figs. 2 and 3, as well as the radiating dark- 
coloured tubes of Fig. 1, are spaces between the concretions 
collected around and incrusting the cell-filaments. 
3. The ‘ cellular ’ structure of sections 4 and 8, the network 
of tubes in the medulla and cortex of sections 5 and 6, and 
the system of white crystalline tubes seen in specimens viewed 
under low power by reflected light, are concretionary deposits 
around the original living cellular framework. 
4. This cellular framework of the plant has, with few ex- 
ceptions, entirely disappeared ; and the tubes left behind are 
usually four or five times larger than the original cells. In a 
few cases the longitudinal walls may be traced (see Figs. B 
3 and 5, and perhaps specimen 7). More frequently the 
transverse walls alone are preserved. 
5. The transverse walls appear in surface view as a pair 
of sharply defined concentric circles, remarkably like the 
M2 
