130 THALLOPHYTA. [CH. 



cases the organism has given rise to oolitic spherules, which in 

 radial section exhibit the branched tubular cells spreading 

 in fan-shaped groups from the centre of the oolitic grain. The 

 section parallel to the surface of a nodule presents the appearance 

 of a number of circular or elliptical tubes cut across transversely 

 or more or less obliquely. The resemblance between the fossil 

 and a specimen of the recent species Zonatrichia calcivora 

 Braun, is certainly very close, but it is very difficult, in the 

 absence of material exhibiting more detailed structure than is 

 shown in the specimens described by Bornemann, to decide with 

 any certainty the true position of the fossil. The figures do not 

 enable us to recognise any trace of cells in the radiating tubes. 

 It is possible that we have in ZonatricMtes an example of a 

 Cyanophyceous genus in which only the sheaths of the fila- 

 ments have been preserved. In any case it is probable that this 

 Mesozoic species affords another instance of a fossil alga which 

 has been responsible for certain oolitic or other structures in 

 limestone rocks. 



The species described by Bornemann was obtained from a 

 Breccia near Lissau in Silesia, of Keuper age. 



M. Renault has recently described certain minute structures 

 in a Palaeozoic coprolite to which he gives the name Gloioconis 

 Borneti^, and which he regards as a Permian gelatinous alga 

 similar to the well-known recent genus Gloeocapsa. The appear- 

 ances revealed in a section of the coprolite are interpreted by this 

 author as a collection of small colonies of a unicellular gelatinous 

 alga in various stages of development. Renault's figure shows 

 a spherical group of faintly outlined and cloudy bodies, most 

 of which include one or two small dark spots. The latter are 

 regarded as the cells of the alga, and the surrounding cloudy 

 substance is described as the gelatinous sheath. The absence 

 of a nucleus in these extremely minute fossil cells (8 — 10 fx in 

 diameter) is referred to as an argument in favour of referring 

 the organism to the Cyanophyceae rather than to the Chloro- 

 phyceae. It is possible that the ill-defined structure described 

 by Renault may be a petrified alga, but there is not sufficient 

 evidence to warrant a decided opinion ; the absence of nuclei 

 ' Benault (96i) p. 446. 



