THE SXOUT OF XIPHT.AS GLADXUS. 323 



trabeculte of bone (tr). When the teeth first erupt they are 

 slightly movable and easily detached, breaking away at the 

 point of junction of tooth and pediment, bat rapidly they become 

 so firmly anchylosed that considerable force is required to detach 

 a tooth. The line of fracture, however, is still the same. 



In PI. III. fig. 2, a developing tooth of a Sea-Bream is shown in 

 which the relationship of the various parts is seen to be similar 

 to tl\at described in the Blenny. Tlie continuity of the cells on 

 the surface of the dentinal papilla (d.p) is well shown. I huxe 

 followed out a complete cytomorphosis of these cells in several 

 Teleostean fishes and find that the changes through which they 

 pass are identical whether they go to form the denticle, the 

 pediment, or the connecting a,rea, 



PI. III. fig. 3, exhibits a transverse section through the pedi- 

 ment of a recently erupted jaw-tooth of a Sea-Bream. It is seen 

 that trabeculse (tr) pass outward from the surface of the pediment 

 (p) and, traversing the connective tissues, blend with those of 

 adjoining teeth already fixed in place. These trabeculpe ai'e at 

 first thin, flexible, and transparent, but rapidly thicken until 

 -eventually a section through this ai-ea presents the appearaiice of 

 an almost solid plate of bone. 



Once firm attachment is efiected the pediment in its growth 

 downwards no longer retains its annular form but divides into 

 i^eparate trabeculae, the bone cells on the outer surface being 

 continuous with and indistinguishable from those on the inner 

 surface and derived from the dental papilla. The joulp chamber 

 of the erupted tooth becomes almost completely obliterated, the 

 typical dentinal structure being maintained, whilst coarse tra- 

 beculse grow inwards and fill the lumen of the pediment. 



Briefly put, the tooth with its pediment develops deep in 

 the tissues as a single entity, and not until it has assumed its 

 functional position does its pediment enter into connection with 

 those of adjacent teeth to form a continuous supporting plate ; 

 the tooth invariably retains its charactei'istic dentinal structiu'e 

 but the pedinient in its growth undergoes a gradual and pro- 

 gressive ti-ansition, both on its outer and inner surface, from 

 dentine to bone and becomes incorporated in the substance of 

 the bone. 



The translucent zone (tz) constituting the junction of tooth and 

 pediment, though formed before tlie eruption of the tooth, calcifies 

 latest and usually appears to have a glassy structure. 



The mode of development described above for the teeth holds 

 good for the denticles in the young Xiphias, for though the 

 soft tissues are somewhat macerated, my niaterial being from 

 a specimen preserved in spirit, yet the developing denticles 

 which have not yet entered into any connection with the bone 

 are found to consist of a cap of dentine and a pediment, connected 

 by a transparent area, and, later, tiubecul* may. be seen formiiig 

 to join adjacent pediments. These pediments are seen to stand up 

 iibove the level of the surrounding bone, but this bone continues 



Proc. Zool. Soc— 1919, No XXIII. 23 



