582 MRS M. F. ROMANES ON 



The tubuli of this organism form the centres of all the nodules, and can be dis- 

 tinguished amongst the fine calcareous material, in association with the quartz grains, 

 etc., which form the nuclei of the nodules. The average diameter of the tubuli is 6/x. 

 Their length it is not possible to measure, as they twist and bend, the same cell- 

 thread passing to different levels in the section. 



Sometimes they appear to branch dichotomously. They form an anastomosing 

 bundle of cell-threads in the centre of the nodules, and have no concentric arrange- 

 ment till towards the outside, and here, owing to the state of preservation, it is 

 frequently impossible to distinguish them. The boundary of each pisolitic grain is, 

 however, clearly denned against the cementing matrix. In every respect the tubuli 

 are comparable with the smaller varieties described by Mr Wethered * in his papers 

 on Girvanella. The dimensions of the tubuli in these species are as follows : — 

 G. minuta, 7/J-; G. intermedia, 10/x ; G. incrustans, 10/u,. The first two occur in the 

 Coralline oolite of the Jurassic ; and the third, together with G. sinensis, described 

 by H. YABE,f and having tubuli 10> in diameter, is confined to the Carboniferous. 

 This species from the Albian beds of Angola is, therefore, the smallest so far described, 

 and may be known as Girvanella minima, n. sp. 



The genus is now accepted as a calcareous alga, although its systematic position 

 has not yet been definitely ascertained. There is reason to believe that it is most 

 nearly related to the Siphonese.ij; So far, Girvanella has been described as ranging 

 from the Cambrian to the Jurassic ; but, as Mr Chapman remarks, " others, no doubt 

 allied to fossil forms, exist at the present day." It is not surprising to find it in 

 Cretaceous rocks which have been deposited under suitable conditions. 



The second genus has been identified as Lithothamnion Phil. This alga occurs 

 towards the circumference of the nodules, frequently as little excrescences, and can 

 be detected on nodules that have been freed from the matrix. In longitudinal section 

 the cells have a width of from 8/a-12/a, and their length is greater than this; but 

 measurement is difficult, as there is reason to believe that the transverse walls have 

 suffered more by replacement than the longitudinal walls. The quadratic form of 

 the cells, characteristic of Lithothamnion ,§ is not so clearly developed as in some 

 species, but can be made out, and the lack of clearness is almost certainly due to the 

 state of preservation. The thallus is made up of cells, somewhat radially arranged ; 

 branching takes place at different levels, so that the whole assumes a fan-shaped 

 form growing from a point. The edges, where growth has been last in process, 

 are irregular and lobate. This applies equally to sections cut tangentially near the 

 growing point. 



The measurements of the cells in tangential sections vary from 8/u, x 10/x to 



* E. Wethered, Q.J.G.S., vol. xlvi, 1890, p. 270, "On the Occurrence of Genus Girvanella in Oolitic Rocks" ; 

 also Geol. Mag., dec. 3, vol. vi, 1889, p. 196, " On the Microscopic Structure of Jurassic Pisolite." 

 t H. Yabe, op. cit. 



\ F. Chapman, Rep. Aust. Ass. Adv. Sci., vol. xi, 1907, p. 377. 

 § A. Rotupletz, Ztit. deutsch. Geol. Ges., vol. xliii, 1891, p. 302, "Calcareous Algae." 



