452 DR. C. I. FORSYTH MAJOR ON 



tire not in all cases so evident, and so surprising at the same time, as in the group under 

 consideration, or as in Geomyidre *, or Kaplodonfia f , it is nevertheless a fact that neither 

 in Rodents nor in Mammalia generally is the surface of the crown absolutely identical 

 throughout its length ; although in many of them we may sj)eak of a relative constancy 

 of pattern. 



Hensel, in the description of the upper teeth of Frolagns, has overlooked this 

 circumstance, and as a result has in one case wrongly interpreted the tooth-structure. 

 This occurs in the description of " Myolugus sardus ; " % but, since Fraas has endorsed 

 Hensel's error in his description of Prolagus a'ti/ngensis (Kon.) {'' Mi/olaffus Mei/eri") §, 

 which differs very little from the former, we shall have to deal with the argument in the 

 present description as well. How little both Hensel and Fraas n-ere aware of tbe change 

 of pattern depending on the age of the animal is shown by the way in which, for 

 convenience sake, they studied the tooth-crown. Hensel does not figure tlie natural 

 surface of attrition > but gives transverse sections of it ||; while Fraas declares % that it is 

 more convenient to examine the teeth from the inferior side, meaning the open alveolar 

 end of the shaft ! 



Fig. 21, PL 36, rejiresents the four upper grinding-teeth of Prolagus ceningensis in a 

 rather worn condition. Both the upper true molars, the fourth and fifth in the series, 

 those teeth which in Titanomys exhibit a beginning of reduction on the postero-external 

 side, have undergone in Prolagus ceningensis a considerable change as compared with 

 the same teeth in the former genus. Of the two more or less crescentic enamel folds 

 of Tikmomys, only one, apparently the inner, persists, in the foi-m of a very small 

 enamel islet in the posterior part of the triturating surface {b). The notch of the internal 

 side (ff) has been transformed into a transverse enamel fold, which, as we shall find to 

 be likewise the case in Lagomys and Lejyns, approaches the outer side of the tooth. The 

 enamel lining of the outer side, partially interrupted in the postero-external corner of 

 m. 2 of Titanomys, is almost entirely missing in the external border of both the molars 

 of Prolagus (and of its posterior premolar as well). In other words, the outer parts of 

 the crown, those which are the least affected by trituration, have degenerated in conse- 

 quence of disuse; and avc might be inclined to assume that com2)ensation has been 

 effected by the transverse fold penetrating towards the outer part. But this is not, to all 

 appearance, the exact explanation of the phenomenon. The tritui'ating surface in the 

 tooth of the young animal — in the part of the shaft which is the earliest formed — is 

 more square than in the adult ; in the latter, it presents the well-known narrow 

 transverse shape of the lagomorphine upper molar. If we remove one of these teeth 

 from its socket and examine it from the anterior or posterior side, it can be seen that, 



* 0. Hurt Merriam, ' Monographic lievisioii of the Pocket Go[iliers, Family Geomyidai ' (North American Fauna, 

 no. 8), pi. 16 (1895). 



t Proc. Zuol. Soe. Loudon, p. 70(i (1807). 



+ Zeitschr. deutsch. geol. Ges. viii. pp. 690, 691 (1856). 



§ Wiirtt. uaturw. Jahresh., xxvi. pp. 174, 17-^ (1870). 



II " Die Baekenziihno siiid stets senkrecht zu ihrer Axe angeschliffen worden, daher sind die Abbildungen 

 eigentlich eine Aneiuanderreihuug der einzelucu Uuerschnitte " (I. c. p. 703). 



^1 Oi>. fit. p. 173. 



