882 



TEETH. 



all the processes of the upper dental plate 

 when the mouth is shut, whilst only the four 

 anterior processes are reciprocally received 

 into the notches of the upper dental plate, 

 this, with the supporting arch, being anterior 

 to the lower plate, a position which is deci- 

 sive in favour of its maxillary character, and 

 against its homology with the vomer. 



The dental plate consists, as in the Cod 

 and SphyrcEna, of a central mass of coarse 

 osseous substance, traversed by large and 

 nearly parallel medullary canals, and an ex- 

 ternal sheath of very hard "vitro-dentine." 

 The medullary canals are continued from a 

 coarse reticulation of similar but wider canals 

 in the substance of the supporting bone, and 

 advance forwards, nearly parallel with each 

 other and with the plane of the upper surface 

 of the tooth ; they anastomose together by 

 short, curved, transverse canals, which inter- 

 cept spaces increasing in length as the canals 

 recede from the osseous basis. The canals 

 themselves diminish in size in the same ratio ; 

 and when they have arrived near the dense 

 outer layer, their divisions and inosculations 

 become again more frequent, the peripheral 

 loops forming a well-marked line of demarca- 

 tion between the coarse-tubed and the fine- 

 tubed dentine. The interspaces of the 

 medullary canals are occupied by a clear sub- 

 stance, and by moss-like reticulations of fine 

 dentinal tubes, which appear to be more 

 sparing in number than in the teeth of the 

 Sphyrcena or Shark. The dentinal tubes of 

 the vitro-dentine run nearly parallel to each 

 other, and vertically to the external surface of 

 the dental plate through about two-thirds of 

 the thickness of that tissue ; they then bend 

 and cross each other in a manner very similar 

 to those of the vitro-dentine in the teeth of 

 the Lepidotus, Phyllodus, &c.* 



In the process of attrition this external 

 dense substance is worn away from the upper 

 surface of the dental processes in the lower 

 jaw, exposing the softer vaso-dentinal sub- 

 stance of the tooth ; in this state the den- 

 tal plate offers an analogy to the incisors 

 of the Rodents, a posterior softer substance 

 being sheathed by an anterior denser layer ; 

 and an external sharp edge is similarly kept 

 up by the unequal wearing away of the two 

 substances. The progressive waste at the 

 upper surface of the dental plate would appear 

 to be met by a corresponding additon of new 

 material to its lower part. 



In the structure here presented we have a 

 condition of the dentine which has hitherto 

 been met with only in the class of fishes. 



The test of the affinities of the present 

 paradoxical genus, afforded by the micro- 

 scopic examination of the teeth, gives addi- 

 tional confirmation to the view to which I 

 have been led, from arguments drawn from 

 the rest of its organisation, that the Lepido- 

 siren is in every essential point a member of 

 the class of fishes.-|- 



* See Odontography, pp. 70. 166, pi. 59,,/fy. 4. 

 f Linnean Transactions, vol. xviii. 1839. p. 350. 

 That the large size, or elliptical form of its bloocl- 



Dental System of Reptiles. 



If we compare the dental system of the 

 foregoing Batrachoid fish with that in the 

 true Batrachia, it is only to the larval state 

 of the Anourans that an analogy can be 

 found ; the tadpole of the frog having its 

 maxilla and mandibula each sheathed with a 

 single and continuous horny dental trenchant 

 covering. Were this sheath actually dentinal 

 in tissue and united to the jaw-bone, the re- 

 semblance to the Lepidosiren would be closer ; 

 but in point of fact the analogy is very 

 remote; the horny beak of the tadpole is 

 never calcified or anchylosed, but is shed 

 during the progress of the metamorphosis.* 

 The Siren alone, among the larval-like pe- 

 rennibranchiate reptiles, retains the sheath 

 upon the extremity of the upper and lower 

 jaws ; it consists of a firm albuminous tissue, 

 and becomes harder than horn. But these 

 trenchant mandibles, which play upon one 

 another like the blades of a pair of curved 

 scissors, are associated with numerous small 

 but distinct true teeth, which are grouped 

 together to form a rasp-like surface on each 

 half of the divided vomer, and which beset 

 the alveolar border of the splenial element of 

 the mandible below. 



In the class Reptilia, the whole order of 

 Chelonia is edentulous, as well as the whole 

 family of Toads (Bufonidce) in the order 

 Batrachia; certain extinct genera of Sau- 

 rians were likewise edentulous, e.g. the re- 

 markable " Rhynchosaurus " of the new red 

 sandstone of Shropshire, and some of the 

 extinct Saurians of South Africa. 



In the tortoises and turtles the jaws are 

 covered by a sheath of horn, which in some 

 species is of considerable thickness and very 

 dense ; its working surface is trenchant in the 

 carnivorous species, but variously sculptured, 

 and adapted for both cutting and bruising in 

 the vegetable feeders ; it may be said that the 

 transitory condition of the mandibles of the 

 Batrachian larvae is here persistent. 



The development of the continuous horny 

 maxillary sheath commences, as in the parrot 

 tribe, from a series of distinct papillae, which 

 sink into alveolar cavities, regularly arranged 

 (in Trior.yx} along the margins of the upper 

 and lower jaw-bones : these alveoli are in- 

 dicated by the persistence of vascular canals 

 long after the originally separate tooth-like 

 cones have become confluent, and the horny 

 sheath completed. 



The teeth of the dentigerous Saurian, 



discs should outweigh the cumulative evidence esta- 

 blishing the piscine nature of the Lepidosiren could 

 only be surmised by those who are ignorant of the 

 variation in size and shape which the blood- discs 

 present in the class of fishes, and the consequent 

 unimportance of those particles as a character of the 

 class. As well might the Petromyzon be deemed a 

 mammal because its blood-discs are circular and 

 comparatively small, as the Lepidosiren be held to 

 be a Batrachian because its blood-discs are elliptical 

 and comparatively large. 



* The large dental plates of Lepidosiren have 

 their nearest homologues in those of the extinct 

 fish called Ceratodus (Odontography, pi. 22,/#.2.). 



