THE SNOUT OF XIPHIAS GLADIUS. 325' 



degree of motion in every direction. The tooth is apparently 

 held in its place by a capsular expansion of the membrane which 



covers the surface of the scale It only requires the 



tooth to be fixed instead of movable, and depressed instead of 

 acuminate, in order to render it the exact homologue of one of 

 the areolte in the cosmine of MegcdichthysJ' 



Since tooth or denticle, pediment, and connecting area are 

 formed on the surface of the same papilla, it seems certain that 

 the difference of structure seen in the fully developed hard 

 tissue is due to some influence exercised by the investing tissues. 

 Over the area invested by epithelium the typical dentinal 

 structure obtains ; where the investment is mesodermal there i& 

 an approximation to bone, whilst at the point of junction of 

 epithelium with mesoderm a layer is formed which, when 

 calcified, presents a glassy appearance. 



Though I have advanced evidence to show that the bone to 

 which the teeth or denticles are attached is not independently 

 developed, but is "an extension of the denticle cone," so that the 

 sharp line of demarcation drawn by Goodrich between the tooth- 

 bearing bones in Teleostei and the bases of Placoid scales does not 

 exist, 3'^et I have refrained, purposely, from discussing the very 

 interesting theoretical problems Avhich arise from a comparison of 

 the conditions described in this paper with those existing in other 

 orders of fishes. Until we possess a satisfactory account of the 

 process of histogenesis of bone and dentine in fishes, based on 

 properly fixed material, we cannot answer certain essential 

 questions. Only the possession of fresh facts can advance oin^ 

 knowledge beyond the position so admii^ably presented in the 

 classical papers of Williamson and of Goodrich. 



My cordial thanks are due to Mr. F. J. Pittock, of the 

 Zoological Department of University College, for the j)hotogTaphs 

 used to illustrate this communication. 



EXPLANATION OF THE PLATP]S. 



The letters used linve tlie following significance : — 



d, denticle. ^h pediment. 

 d.p, dentinal papilla. pm.i; premaxilla. 



e, epithelium. tr, trahecula-. 



m, mesoderm. #- translucent zone. 



Pi ATE I. 



Fig. 1. Xipliias gladius. The bone de\ eloped by the junction of trabeculic {tr) 

 connecting the pediments {p) is seen blending with the surface of the 

 premaxilla (pw.v). X90. 



Fig. 2. The same. The circular pediments {p) are seen connected one with another 

 bj' means of trabecube (tr) which also attach them to the neighbouring 

 bone. X 40. = & 



Fig. 3. The same. On the right-hand side of the figure denticles {d) are seen 

 developing in the soft tissues ; denticle [d] and pediment ( ■p) are seen 

 connecting by a ti-anslucent zone [tz). X 40. 



23* 



