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Summary.

1. The Carboniferous Cælacanths of the British Isles have been re-examined and together with the foreign forms have been rediscussed.

2. Although many of the more fragmentary species may belong to another genus, all have been included in the single genus Rhabdoderma, which has been distinguished from the Permian genus Coelacanthtis.

3. The anatomy of RhaModerma has been redescribed and the following new points described for the first time:–The presence of a suboperculum, the coronoid, the ectopterygoid, the dermo- and auto-palatines, the shape of the basisphenoid olico-occipital and ethmoid region and the nature of the circumorbital plates, and the course of the sensory canal. The pectoral and pelvic girdles and internal skeleton of the pelvic fin. The skeleton of the anterior dorsal fin and its articulation with the neural spines in some forms. The internal skeleton of the anal fin. The presence of basiventrals in the vertebral column.

4. The following species have been considered synonymous with R. elegans: “C.” ornatus Newb., “C.” robustus Newb., “C.” summiti Wellb., “C.” elongatus Hux., “C.” watsoni Aiding., “C.” arcuatus Hibb. & “G.” newelli Hibb.

5. “C.” mucronatus Pruvost is considered synonymous with R. tingleyense.

6. “Ccelacanthopsis curta” Traq. is shown to be a peculiarly preserved, but normal Cælacanth. It is considered specifically and generically indeterminable.

7. The following new species have been described:–R. ardrossense, R. (?) aldingeri, R. (?) abdenense, R. (?) davisi.

8. Details of anatomy have been described in R. elegans, R. tingleyense, R. huxleyi, R. corrugatus.