'o 



THE KEPIIODUCTION OF (nilUTOLlTES. 347 



EXPLANATION OF PLATP: XV. 



Fig. 1.— Cuff's Wire Micrometer. 

 2. — Baker's Hair MicroiiicLer. 



3.- Adam's Needle Micrometer. A, B, C, the sectional scale. 

 4. — Zeiss' Stage Micrometer. 

 5. — Jackson's Eye-piece Micrometer. 

 6. — Jackson's Micrometer Eye-piece. 

 7. — Ramsden's Micnuneter Eye-piece. 

 8. — The scale of the same. 



©11 the 1Rcpro^uctiou of iS)rbitoltte9/'' 



By J. J. Lister, M A. 



MR. H. B. Br.\dy has described specimens of Orbitolites 

 which he obtained in Fiji, showing the margin of the disc 

 crowded with young shells. Mr. Brady's material was 

 worked at in the dry state, and it was at his suggestion that the 

 author collected specimens, preserved in spirit, from the Tonga 

 reefs. Examination of this material shows that large brood- 

 chambers are formed at the margin of the disc during the later 

 stages of growth. These are, at first, lined with a thin layer of 

 protoplasm At a later stage the central region of the disc is found 

 to be empty, and the whole of the protoplasm is massed in the 

 brood-charabers in the form of spores. The spores have the 

 structure of the " primitive disc," which, during the early stages 

 of growth of the Orbitolites, occupies the centre of the shells. 

 They are liberated by absorption of the walls of the brood- 

 chambers, and each becomes the centre of a new disc, which is 

 built up by additions of successive ridges of chamberlets at the 

 margin. The reproduction of Orbitolites, therefore, takes place 

 by spore formation. 



The spore contains a single nucleus, lying in its " primordial 

 chamber." After several rings of chamberlets have been added, 

 a stage is reached at which the nucleus appears to be represented 

 by numbers of irregular, darkly staining masses, scattered through 

 the protoplasm of the central part of the disc. 



In the later stages, numbers of oval nuclei are found in the 



protoplasm, often arranged in pairs, and in favourable preparations 



they may be seen to be undergoing amitotic division. 



* Extracted from the Proceeding's of the Cambridge Philosophical Society 

 by G. H. Bryan. 



