part 3] AXD STKUCTURE or THE PORAMIIN^IFEEAL SHELL. 199 



Of the existence of an intraskeletal organic network there can, 

 however, be no doubt ; but the amount of matei'ial that it contri- 

 butes to the skeleton is comparatively small, and it Avould be 

 scarcely appropriate to speak of it as an organic basis, although 

 this terra may be conveniently employed to designate the skeleto- 

 genous layers : that is, the lining membrane and included network 

 taken together. 



Attempts made to determine the specific gravity of the organic 

 residue by means of a diffusion-column were met by unexpected 

 difficulties : the substance of this residue Avas apt to swell up, to 

 become ' sticky ' and lose its consistency, and the results which 

 were obtained w^ere so divergent as to be of no scientific value, 

 except as showing that the organic basis of the shell cannot be 

 chitin : but this was alread}'' known, for Awerinzew in a very 

 valuable paper has not only shown that the basis cannot be chitin, 

 but has referred it to the albuminoids ^ and indeed identified it as 

 a keratin." He also calls attention to the fact that the properties 

 of keratin, as of all albuminoids, are affected by age, and thus 

 furnishes an explanation of the unsatisfactory results which were 

 obtained from my specimens, for they were none of them freshly 

 collected, and some were very old indeed. It ma)" be of interest to 

 add here that an exauiination under the microscope of the organic 

 basis afforded by these specimens revealed the presence of various 

 impurities, in particular an abundance of minute diatoms of more 

 than one kind. These were removed with fiuoric acid ; but this 

 treatment caused the keratin to swell up more readily than before, 

 and rendered hopeless an}^ attempt to determine its specific 

 gravity. 



Minute Structure of the Imperforate Shell. 



When a thin section of OrhitoJifes is examiiied under a fairly 

 high magnification, such as a No. 7 Fuess, a well-marked fibrillar 

 structure is seen, the fibrils running more or less concentrically 

 round each chamber (PI. VII, figs. 1 & 3). This structure was 

 first seen and figured by Rhumbler^ in specimens etched with 

 picric acid ; such preliminary treatment is, however, quite super- 

 fluous, for the structure is obvious enough without etching, provided 

 that the sections are sufficiently thin. Rhumbler does not pursue 

 tlie matter further, except to make the inapposite remark that 

 his observation is confirmatory of Awerinzew's description of the 

 crystalline structure of the shell. 



In horizontal sections of Orhitolites the chambers may be de- 

 scribed as bounded distally by arches, and at the sides by the piers 

 of these arches, wdiich in turn rest on the crown of the arches of 

 the zone next succeeding towards the centre. The proximal 



^ S. Awerinzew, op. cit. 1903, p. 482. 



2 Id. Mitt. Zool. Stat. Neapel, vol. xvi (1904) p. 349, in particular p. 356. 

 ^ 'Die Foraminiferen der Plankton-Expedition' vol. iii, pt .1 (1913) p. 103 

 & fig. 29. 



