SHARKS. 35 



to the apex of the tooth, the lateral ones also run parallel to each 

 other, and to the axis of the tooth, at the beginning of their course, 

 and then bend gently outwards to the margins of the tooth. The 

 secondary curvatures of the medullary tubes are pretty regular, and 

 of an angularly undulating character ; the whole of the clear outer 

 enamel-like covering consists of parallel and extremely minute calci- 

 gerous tubes with intermixed cells. The calcigerous tubes, in the 

 body of the tooth, are given off at an acute angle from the medullary 

 tubes through their whole course. 



It has been already observed that the formation of the teeth of 

 the sharks, as of many other fishes, exemplifies, on a large scale, the 

 earliest or papillary stage of dental development in the higher classes 

 of animals. It is not succeeded here by either a follicular or an 

 eruptive stage ; the formative papillae are never inclosed, and con- 

 sequently never break forth. The pulp, when consolidated by the depo- 

 sition of the calcareous salts in the pre-existing cells and tubes, is 

 gradually withdrawn from the protective sheath which the thecal fold 

 of mucous membrane afforded it during the early stages of its forma- 

 tion. I have studied the development of the teeth of the squaioids, 

 in the genera Galeus, Carcharias, and Scymnus. In the uterine foetus, 

 one foot long, of the great white shark, {Carcharodon) the jaws seem at 

 first sight to be edentulous ; a fissure presents itself on the inner side 

 of the margin of each jaw running parallel with it, between the thin 

 smooth membrane covering the convex edge of the cartilage, and the 

 free margin of a fold of mucous meiiibrane which lies parallel to, and 

 upon the inner side of the jaw. When this fold is drawn away from 

 the jaw, the minute teeth are exposed, arranged in the usual vertical 

 rows ; their points are all directed backwards and towards the base 

 of the jaw, and are seen to slip out of fossae, or sheaths in the 

 membranous fold, as this is gradually reflected backwards to 

 its line of attachment near the base of the jaw. Here the an- 

 terior lamina of the fold, which, from its office may be 

 termed *' thecal," is continuous with the mucous membrane at the 

 base of the rows of teeth ; the posterior layer is reflected back- 

 wards to the frsenal line of attachment of the tongue. Close to the 

 anterior line of reflection there is a row of simple conical papillee, in 



D 2 



