VIl] PILA. 181 



walls are fairly thick. In some of the larger specimens there 

 are often found a few smaller sacs enclosed in the cavity of the 

 partially disorganised mother-thallus. In the larger specimens 

 the wall is usually invaginated in several places, giving the 

 whole thallus a lobed or brain-like appearance. The supposed 

 alga, which makes up j%ths of. the contents of a block of 

 Kerosene shale, is named Reinschia Australis ; it is regarded 

 by the authors of the species as nearly related to the Hydro- 

 dictyaceae or Volvocineae. 



In the Kerosene shale from certain localities in New South 

 Wales Bertrand recognises a second form of thallus, which he 

 refers to the genus Pila, characteristic of the European Bogheads. 



Pila. Fig. 36, 2. 



The "thallus" characteristic of the Scotch Boghead has been 

 named Pila scotica, and that of the Autun Boghead, Pila 

 hibractensis. 



In the latter form, which has been studied in more detail 

 by MM. Renault and Bertrand, the thallus consists of about 

 6 — 700 cells, and is irregularly ellipsoidal in form, from "189 — 

 •225mm. in length, and '136 — 'IBOmm. broad. The surface- 

 cells are radially disposed and pyramidal in shape, the internal 

 cells are polygonal in outline and less regularly arranged 

 (fig. 36, 2). The Pila thalli make up fths of the mass in an 

 average sample of the Autun Boghead. The Autun Boghead 

 often contains siliceous nodules, and sections of these occasionally 

 include cells of a Pila in which the protoplasmic contents and 

 nuclei have been described by the French authors. The evidence 

 for the existence of these supposed nuclei is, however, not entirely 

 satisfactory ; sections of silicified thalli which were shown to 

 me by Prof. Bertrand did not satisfy me as to the minute 

 histological details recognised by Bertrand and Renault. 



The species of Pila are compared with the recent genus 

 Celastrum, and regarded as most nearly allied to the Chroococ- 

 caceae or Pleurococcaceae among recent algae. Prof. Bornet' 

 has suggested Gomphosphaeria as a genus which presents a 

 resemblance to the Autun Pila, 



1 Bertrand and Renault (92) p. 29. 



