880 



TEETH. 



and anchylosed to, the processes of the sup- 

 porting bone, this likewise enters into the 

 formation of the masticatory surface when 

 the tooth is worn down to a certain point. 



The proof of the efficacy of the complex 

 masticatory apparatus above described is 

 afforded by the contents of the alimentary 

 canal of the Scari. Mr. Charles Darwin, 

 the accomplished naturalist and geologist, 

 who accompanied Captain Fitzroy, R. N., in 

 the circumnavigatory voyage of the " Beagle," 

 dissected several Parrot-fishes soon after they 

 were caught, and found the intestines laden 

 with nearly pure chalk, such being the nature 

 of their excrements ; whence he ranks these 

 fishes among the geological agents to which 

 is assigned the officeof converting the skeletons 

 of the Lithophytes into chalk. 



Development. As might have been antici- 

 pated from the discovery of the varied and pre- 

 dominating vascular organisation in the teeth 

 of fishes, and the passage from non-vascular 

 dentine to vascular dentine in the same tooth, 

 the true law of the development of dentine 

 "by centripetal metamorphosis and calcifica- 

 tion of the cells of the pulp," was first defi- 

 nitely enunciated and illustrated from observ- 

 ations made on the development of the teeth 

 of fishes.* 



It is interesting to observe in this class the 

 process arrested at each of the well-marked 

 stages through which the development of a 

 mammalian tooth passes. In all fishes the 

 first step is the simple production of a soft 

 vascular papilla from the free surface of the 

 buccal membrane : in Sharks and Rays these 

 papillae do not proceed to sink into the sub- 

 stance of the gum, but are covered by caps of 

 an opposite free fold of the buccal membrane ; 

 these caps do not contract any organic con- 

 nection with the papilliform matrix, but, as 

 this is converted into dental tissue, the tooth 

 is gradually withdrawn from the extraneous 

 protecting cap, to take its place and assume 

 the erect position on the margin of the jaw 

 (fig. 510, b, a, art. PISCES, Vot. III. p. 976.). 

 Here, therefore, is represented the first and 

 transitory " papillary " stage of dental develop- 

 ment in Mammals ; and the simple crescentic 

 cartilaginous maxillary plate, with the open 

 groove behind containing the germinal papillre 

 of the teeth, offers in the Shark a magnified 

 representation of the earliest condition of the 

 jaws and teeth in the human embryo. 



In many fishes, e. g. Lophius, Esox, the 

 dental papillae become buried in the membrane 

 from which they rise, and the surface to which 

 their basis is attached becomes the bottom of 

 a closed sac : but this sac does not become 

 inclosed in the substance of the jaw ; so that 

 teeth at different stages of growth are brought 

 away with the thick and soft gum, when it is 

 stripped from the jaw-bone. The final fix- 

 ation of teeth, so formed, is effected by the 



* In my Hunterian Lectures, delivered at the 

 Royal College of Surgeons, May, 1839. See also, 

 Compte Rendu de l'Acade'mie des Sciences, Dec. 

 1839, p. 784. ; and Odoutography, Introduction, and 

 part i. passim. 



development of ligamentous fibres in the sub- 

 mucous tissue between the jaw and the base 

 of the tooth, which fibres become the medium 

 of connection between those parts, either as 

 elastic ligaments, or by continuous ossifi- 

 cation. Here, therefore, is represented the 

 " follicular " stage of the development of a 

 mammalian tooth ; but the " eruptive " stage 

 takes place without previous inclosure of the 

 follicle and matrix in the substance of the 

 jaw-bone. 



In Batistes, Scarus, Sphyrama, the Sparoids, 

 and many other fishes, the formation of the 

 teeth presents all the usual stages which have 

 been observed to succeed each other in the 

 dentition of the higher Vertcbrata: the papilla 

 sinks into a follicle, becomes surrounded by a 

 capsule, and is then included within a closed 

 alveolus of the growing jaw (Jig- 516. r, 

 Vol. III. p. 979. art. PISCES), where the de- 

 velopement of the tooth takes place and is 

 followed by the usual eruptive stages. A 

 distinct enamel-pulp is developed from the 

 inner surface of the capsule in Batistes, Scarus, 

 Sargus, and Chrysophrys. 



The most formidable dentition exhibited in 

 the order of osseous fishes, is that which cha- 

 racterises the Sphyrccna, and some extinct 

 fishes allied to this predatory genus. In the 

 great Barracuda of the southern shores of the 

 United States (Sphyrccna Barracuda, Cuv.), 

 the lower jaw contains a single row of large, 

 compressed, conical, sharp-pointed, and sharp- 

 edged teeth, resembling the blades of lancets, 

 but stronger at the base. The two anterior 

 of these teeth are twice as long as the rest, 

 but the posterior and serial teeth gradually 

 increase in size towards the back part of the 

 jaw ; there are about twenty-four of these 

 piercing and cutting teeth in each preman- 

 dibular bone. They are opposed to a double 

 row of similar teeth in the upper jaw, and fit 

 into the interspace of these two rows when 

 the mouth is closed. The outermost row is 

 situated on the intermaxillary, the innermost 

 on the palatine bones ; there are no teeth on 

 the vomer or superior maxillary bones. The 

 two anterior teeth in each premaxillary bone 

 equal the opposite pair in the lower jaw in 

 size : the posterior teeth are serial, numerous, 

 and of small size ; the second of the two an- 

 terior large premaxillary teeth is placed on 

 the inner side of the commencement of the 

 row of small teeth, and is a little inclined 

 backwards. The retaining power of all the 

 large anterior teeth is increased by a slight 

 posterior projection, similar to the barb of a 

 fish-hook, but smaller. The palatine bones 

 contain each nine or ten lancet-shaped teeth, 

 somewhat larger than the posterior ones of 

 the lower jaw. All these teeth afford good 

 examples of the mode of attachment by im- 

 plantation in sockets, which has been denied 

 to exist in fishes.* 



The loss or injury to which these destruc- 

 tive weapons are liable in the conflict which 

 the Sphyrccna wages with its living and 



* Cuvier, Histoire Naturelle des Poissons, torn. i. 

 p 492. 



