lOUANdDON. 



IOUANODON. 



IM 



r abrasion. 



however UM deep excavation of the remaining fang 

 aks UM progrcM of UM rouonsinnil tooth prepared to 



JLhe "place of the worn-out grinder. At the earlier stage* of 

 a harp edge it maintained at the external part of the tooth 



by of UM sntnnl which covers UM surface of the crown ; the 



prominent ridge* opon that surface give a (tenons contour to tho 

 Buddl. of UM catting edge, whilst it* aides are jagged by the lateral 

 1 1 UM adaptation of this admirable dental instrument to the 

 sad comminution of such tough vegetable food a* the 

 r, and similar plants, which are found buried with the 

 , i* pointed oat by Dr. Buckland with his usual felicity of 

 i in hit Bridgewatrr Treatise,' voL i. p. 246. When the 

 row*) is worn away beyond UM enamtl it present* a broad and nearly 

 boriaontal grinding surface; and now another dental substance U 

 brought into us* to give an inequality to the surface; this is the ossified 

 t of UM pulp, which being firmer than the surrounding 

 , form* a slight transverse ridge in the middle of the grinding 

 Tb* tooth in this stage has exchanged the functions of an 

 for that of a molar, and is prepared to give the final corn- 

 a, or comminution, to the coarsely-divided vegetable matter*. 

 " The marginal edge of the incisive condition of the tooth, and the 

 median ridge of Uw molar stage an more effectually established by 

 UM introduction of a modification into the texture of the dentine, 

 by which it i* rendered softer than in the existing Iguana and other 

 reptile*, and more easily worn away ; this is effected by an arrest of 

 UM calcifying process along certain cylindrical tracts of the pulp, 

 which is tan* continued in the form of medullary canals, analogous 

 to those in UM toft dentine of the Mtyatkrrium'i grinder from the 

 central cavity, at pretty regular intervals, parallel with the calcigerous 

 tab**, nearly to the surface of the tooth. The medullary canals 

 radiate from the internal and lateral sides of the pulp cavity, and 

 are confined to the dentine forming the corresponding walls of the 

 tooth ; their diameter is 1 1250th of an inch ; they are separated by 

 pretty regular intervals, equal to from 6 to 8 of their own diameters ; 

 they sometime* divide once in their course. Each medullary canal is 

 turrounded by a clear space; its cavity was occupied in the section 

 ilsn ilii il by substance of a deeper yellow colour than the rest of 

 UM dentine. Tb* calcigerous tubes present a diameter of l-25,000th of 

 an inch, with interspaces equal to about 4 of their diameters. At the 

 first part of their count near the pulp cavity, they are bent in strong 



undulation*, but afterwards proceed in slight and regular primary 

 curves, or in nearly straight line* to the periphery of the tooth, 

 i viewed in a longitudinal section of the tooth, the concavity of 



UM primary curvature is turned towards the base of the tooth ; the 

 lowest tabs* art inclined towards the root, the rest have a general 

 direction at right angles to the axis of the tooth ; the few calcigerous 

 tabes which proceed vertically to the apex are soon worn away, and 

 can be seen only in a section of the apical part of the crown of an 

 incompletely developed tooth. The secondary undulation* of each 

 tooth are regular and very minute. The branches, both primary and 

 nniUry of the calcigerous tube*, are tent off from the concave aide 

 UM main inflections; the minute secondary branch** are remark- 

 at certain part* of the tooth for their flexuous ramifications, 

 and dOatations into minute calcigerous cells, which 



calcigerous cells, 



place along nearly parallel line* for a limited extent of the 

 of UM main tube*. The appearance of interruption in the 

 coarse of UM calcigerous tubes, occasioned by this modification of 

 their secondary branches, i* represented by the irregularly dotted 

 tracts in UM figure. This modification must contribute with the 

 medullary canals, though in a minor degree, in producing that 

 inequality of texture and density in the dentine, which renders the 

 broad and thick tooth of the lywmotUm more efficient as a triturating 



Teeth of 

 tooui ; . r. teeth farther advanced, snd worn. 



U. ter side of UM tooth, pn^taU- 



dirty peculiar browE 



colour when viewed by transmitted light as in most other teeth ; very 

 minute and scarcely perceptible undulating fibre*, running vertically 

 to the surface of the tooth, are the only structure I have been able to 

 detect The remains of the pulp in the contracted cavity of the 

 completely formed tooth arc converted into a dense but true osseous 

 substance, characterised by minute radiated cells, whose long axis is 

 parallel with the plane of the concentric lamella; which surround the 

 few and contracted medullary canals in this substance. The micro- 

 scopical examination of the structure of the lyuanudun't teeth thus 

 contributes additional evidence of the perfection of their adaptation 

 to the offices to which their more obvious characters had indicated 

 them to have been destined. 



" To preserve a trenchant edge, a partial coating of enamel is supplied, 

 and that the thick body of the tooth might be worn away in a more 

 regularly oblique plane, the dentine is rendered softer as it recede* 

 from the enamelled edge by the simple contrivance of arresting the 

 calcifying process along certain tracts of the inner wall of the tooth. 

 When attrition has at length exhausted the enamel, and the tooth is 

 limits to its function as a grinder, a third substance has been 

 prepared in the ossified remnant of the pulp to add to the efficiency 

 of the dental instrument in its final capacity. 



The size of this giant of the Weald in former ages was enormous. 

 In Dr. Mantell's collection there is a portion of a femur 22 inches in 

 girth in the smallest part. The thigh bone then of the Iguanodon 

 exoeeded in bulk that of the largest elephant, and its length is on 

 good grounds calculated to have been from 4 to 5 feet Dr. Miuitcll 

 carefully compared the bones of the lyuanotlon with those of the 

 Iguana, and by taking an average from eight separate parts of the 

 respective skeletons, he gives the following as the dimensions of the 

 former : 



Feet. 

 Length from Snout to the extremity of the Tail . . 70 



Length of Tail 62J 



Circumference of Body 14) 



The thigh bone of the Iguanodon is considered by Dr. Hantell to 

 be twenty times the size of that of a modern Iguana : but as animals 

 do not increase in length in the same ratio as in bulk, it does not 

 follow that the fyuanwlun attained the length of 100 feet, although it 

 probably approached 70 feet (Buck-land.) On the snout of tliU 

 monstrous reptile was a nasal horn, and its appearance must have 

 realised the wildest poetical fictions of dragons of old. 



Nusl horn of Igtuuiodon, two-thirds of the natural ilse. 



In tho Report above alluded to, Professor Owen gives his reasons 

 for doubting if the lyuanodon was so large an animal as is inferred by 

 Dr. MantelL 



" From the comparison," he says, "which the few connected portions 

 of the skeleton of the Iguanodon enable us to make between tlio 

 bones of the extremities and the vertebral column, it is evident that 

 the hind legs at least, and probably also the fore legs, were longer and 

 stronger in proportion to the trunk than in any existing Saurian. 

 One can scarcely suppress a feeling of surprise that this striking 

 characteristic of the Jguanodon, in common with other JKnosauria, 

 should have been hitherto overlooked ; since the required evidence is 

 only an associated vertebra and long bone of the same individual, or 

 a comparison of the largest detached vertebrae with the longest femora 

 or humeri. This characteristic is nevertheless one of the most 

 important towards a restoration of tho extinct reptile, since an approxi- 

 mation to a true conception of the size of the entire animal could 

 only be made after the general pro]x>rtions of the body to tho 

 extremities had been ascertained. But it in very obvious that the 

 exnggerated resemblances of the Iguanodon to the Iguana have mis- 

 led the polnontologist* who have hitherto published the results of their 

 calculations of the size of the Jyuanodon ; and hence the dimensions 

 of 100 feet in length, arrived at by a comparison of tho teeth and 



