324 MR. J. r. cATiTEU : dkxticles ox 



to thicken find grow until its surface lies nlmost at the level of 

 the transparent areas. 



The denticles in the adult Histiophonis, developing ns they do 

 in the substance of tl^e bone, ninv be said to have become teeth, 

 and the appearances seen in sections from the rostrum confoi-ni 

 Avith the observations described in the Blenny and the Bream. 



Mr. Tate Regan has most generously given to me the post- 

 temporal bone of one of the Loricariidae, an adult Pseudaccmthvs 

 seriatus, the examination of which confirms the account I have 

 given as to the mode of development and attachment of the 

 denticles and their part in the formation of the bone. 



The surface of the bone is studded with denticles, some fnlly 

 erupted and so firmly attached ns not to become displaced in the 

 preparation of ground sections of the whole bone, others but 

 partly erupted and freely movable in their sockets which aie 

 widely open like the denticles in the rostrum of IlisiiopJioriis: 

 (PI. II fig. Ti,d). AYhere denticles have become detached the 

 surface presents an appearance of circular areas with slightly 

 raised edges, connected one with another by radiating trabecuhe 

 and, but for the soft tissues being completely removed, affording^ 

 a picture identical with that seen in PI. III. fig. 3. 



A ground section reveals the denticle, composed of hard tubed 

 dentine with a tip of enamel, lesting on a pediment which on its 

 outer surface merges into the surroundiug bone, whilst towards 

 the pulp chamber tlie structure resembles coarse dentine; the 

 denticle is continued downwards a little way into the cavity of 

 the pediment, or in other words, is slightly socketed. In the 

 anchylosed denticles attachment to the pediment is effected by a 

 layer of almost structureless calcified tissue which occupies the 

 area of junction. 



The condition is identical with that seen in certain of the 

 Gadidfe Avhere the tooth, the pediment, and the connecting 

 substance develop on the surface of the one dentinal papilla in 

 the manner described earlier in this paper. At first the line of 

 termination of the base of the tooth is straight as in PI. III. 

 fig. 1, but gradually the inner surface of the connecting trans- 

 parent zone becomes converted into dentine to fomn the little 

 downward extension seen in sections. Eventually, in certain 

 areas of the mouth, the remainder of the connecting zone calcifies^ 

 and the tooth is firmly anchylosed into place. 



It is interesting to compare this description with that given by 

 Williamson (Phil. Trans. 1851, Part II. p. 659) of the scale o'f 

 Loricaria cataphracta. He wrote : — 



" On the surface of the scale there are numerous small circular 

 cavities which communicate inferiorly with branches from the 

 network of Haversian canals. Each cavity is surrounded b}^ a 

 narrow projecting rim, upon which the flanging shouldeis of the 

 tooth rest, whilst its constricted base is fitted into the enclosed 

 hole, thus producing an arrangement which closely resembles a 

 ball-and-socket joint, and which must allow of a considerable 



