476 Mr. D. W. Devanesen. Development of the Calcareous 



and fuse forming a beak-shaped process somewhat like the beak of an ounce- 

 glass. It will be seen that in the formation of this beak both lamella? take 

 an equal share. When several cones fit into one another, their beaks likewise 

 fit together, thus giving rise to a crest which is the precursor of the carina of 

 the adult tooth. The lower part of the tooth in the imago appears to be 

 devoid of this crest ; only the cones arising later develop the beaks in question. 



The fitting into one another of the cones takes place in such a manner that 

 the brim of an upper cone always projects a small distance from above the 

 brim of the lower cone into which it is enclosed. This fact has a necessary 

 bearing, first, on the growth of a carina on the inner side and, second, on the 

 formation probably of the middle furrow on the outer side of each tooth 

 (Plate 15, fig. 6, OA. and text-fig. 1). 



In the aboral end of the embryonic tooth — the root-end in the adult — one 

 can see all the intermediate stages between a pair of calcareous particles, 

 the simplest condition of a pair of lamella?, and a transparent beaked cone, 

 the most advanced stage of the same (text-fig. 2). As has been already 

 mentioned, the first point of contact and fusion between every two lamella? of 

 a pair is at their apices. The fused apices of one pair unites with those of 

 its predecessor and successor even before the pair itself reaches the stage of 

 a cone. Thus a central rod results to which are attached all the cones and 

 the younger pairs of lamellae. Presumably this rod is the " pars petrosa " of 

 Giesbrecht (1, p. 90). Curiously enough, the first cone does not appear to 

 participate in the formation of this stony part (text-fig. 3). 



It has not been possible to follow closely the next phase in the building of 

 the tooth. The coalescence of the cones appears to proceed centripetally from 

 the " pars petrosa " outwards. The outlines of the cones gradually disappear 

 owing to the fusion of their walls. 



The median furrow of the adult tooth probably marks the place where the 

 suture of the two outer edges of a pair of lamellae is formed. Of this line of 

 junction a greater portion is bound to disappear in the process of coalescence 

 owing to its internal position in the cavity of its predecessor as explained 

 already ; only the small exposed parts of this suture in the several cones 

 probably persist and contribute to the formation of the median furrow. 



On account of the relative position between every two cones, the beaks 

 are, on the inner side, the only exposed parts of the line of junction between 

 the two inner edges of the pairs of lamella?. It must be observed that no 

 indication of a suture, however, is to be found on each beak. These beaks, as 

 has been already described, are the precursors of the carina which is for this 

 reason a composite paired structure. Such is the construction of a tooth so 

 far as one could make out from observations of hard structures. 



