FOSSIL AND KECEXT LAGOMOllPHA, 453 



while its outer border maintains tliroughou.t its height almost a vertical direction, or is 

 even concave, the inner part of the tooth slopes down medially, from below to above 

 (taking into account its natural position in the maxillary). The tooth, therefore, as it 

 continues to grow, extends persistently in a transverse direction ; but this growth takes 

 place chiefly, if not exclusively, towards the internal side ; so that the transformation of 

 the internal notch of the TUanomy s-iooih. into the transverse fold of the true molars 

 of Prolagus is not the result of its extension outward but inward. In other words, 

 new formation takes place in that part of the tooth where there is increased work, 

 •while the outer part — that which is scarcely or not at all affected by trituration — not 

 only remains stationary, but even becomes atrophic. 



On the other hand, since in the more specialized forms, beginning with Prolagm 

 sardiis (PI. 36. fig. 24), the transverse enamel fold reaches almost the outer side in the 

 true molars, it is very possible, and even likely, that secondarily a slight extension 

 outicarcl of this transverse fold takes place; although the outer border of the tooth 

 is nearly lunctionless, its condition, almcst devoid of enamel, would nevertheless effect 

 a too rapid wear of the dentine if some compensation were not ensured. 



The posterior of the three premolars, p. 1, situated between tlie first molar and the 

 second premolar, is intermediate in shape as in position. Both tlie crescentic enamel 

 folds of Titanomijs are preserved in this tooth in the shape of central enamel islets, a 

 much larger internal one {h), with an elongate tmterior horn, and a smaller outer 

 one (6') (fig. 21). The transverse fold {a) which opens on the inner side is much 

 shorter than in the molars ; it is scarcely more than an elongate notch. To put it 

 otherwise, as compared with the molars, p. 1 presents less reduction in its external jmrt, 

 and less new^ formation in regard to the transverse fold starting from the inner side. 

 Exactly the inverse is apparent when we compare p. 1 with the tooth in front of it. 



This latter (p. 2) has triangular contours, with the apex internally, a shorter, slightly 

 convex anterior, and a longer posterior side ; as a consequence, its outer border runs 

 obliquely. In its pattern, this premolar strongly resembles the Titanomi/s-teeth ; instead 

 of a transverse enamel fold we find in it a short notch («) on the inner side, as 

 in Titaiioi)ii/s ; while almost the whole of the cro\\-n-surfaee is occupied by the two 

 crescentic enamel folds [b and t-), with an indication of a minute third one— equally 

 marked in Titanomys — on the antero-exterual corner. The latter is more distinct in 

 youu<;er specimens of Prolagiis (jeii'mijensis (PI. 36. tig. 10, p. 2). The enamel folds 

 alternate with crescent-shaped, pointed cusps. 



On comparison of p. 2 witli p. 1 it becomes at once clear that the main ditference 

 between the two consists in the circumstance that the crescentic enamel folds in the 

 former have become reduced to the condition of enamel islets, their communication 

 with the antero-external margin of the tooth having ceased. AVhen describing p. 1 of 

 Frolagns surdm, in which, as a comparison of our figures shows, this tooth (fig. 24, p. 1) 

 is almost identical with its homologue in P. amingensis, Hensel labours under a strange 

 misconception. He says : — " Das Merkwiirdigste aber an dem Zahn sind zwei isolierte 

 Schmelzcylinder. Sie befinden sieh in dem ausseren und hiuteren Viertel des Zahnes." * 



* Oj}. cit. p. 090. 



