334 



ON THE EXTINCT MAMMALIA OF 



transverse fold or valley, except in the first tooth, in which the fold is anterior. In 

 the progress of v^'earing, the transverse valley is shortened, and its somewhat widened 

 and deepened bottom is at first isolated as a central islet to the triturating surface. 

 The islet is subsequently obliterated; the transverse valley continues to be reduced, 

 and is finally obliterated. The teeth increase in breadth as they advance in age, and 

 the enamel is obliterated externally long before it approaches its termination inter- 

 nally. The steps of this process of change may be readily traced in the series of 

 figures from 14 and 15 to 16. 



In the lower jaw the molars are likewise composed of double columns, which in the 

 first and last teeth, from an early period, ajopear on the triturating surface united only 

 by cemenlum, as represented in figures 17, 19. As the teeth are worn away, the 

 columns become associated by an internal isthmus, and are then separated by a deep 

 transverse valley, as represented in figure 18. Later the internal isthmus gradually 

 widens, and the transverse valley is proportionately shortened and is finally oblite- 

 rated, as represented in figure 20. 



As in the case of the upper molars, the lower ones, as they wear down, are repro- 

 duced, and grow of greater breadth. The extent of increase, however, may not be 

 so great as represented between what are viewed as aged and young specimens above 

 described, for the teeth of the latter at the bottom of the crown exceed very little the 

 diameter at the triturating surface. It would therefore follow, either that the crowns 

 as existing in the young specimens would be completely worn away and replaced by 

 other and broader but continuous ones, or that the older and larger specimens belong 

 to a larger species from the younger and smaller ones. 



The measurements of the specimens are as follow : 



Space occupied by six superior molars, . 

 Space occupied by five anterior superior molars. 

 Space occupied by four intermediate molars. 

 Transverse diameter third upper molar. 



Depth of lower jaw below second molar. 



Space occupied by six lower molars, 



Space occupied by anterior three lower molars, 



Antero-posterior diameter of third lower molar. 



Transverse " " " . 1 li IJ li 



Dr. Hayden's last collection of fossils from the Mauvaises Terres contains a few 

 additional fragments of jaws with teeth of Palceolagus Haycleni. The specimens ex- 

 hibit nothing peculiar from those already described, and are only of especial interest 

 from the fact of their having been derived from a different locality. 



1st spec. 



2dsp. 



Sdsp. 



Lines. 



Lines. 



Lines. 



. 4i 







. 4 







. 3i 



4 



4i 



. li 



li 



2i 



1st sp. 



2d sp. 3d sp. 



4th sp. 



Lines. 



Lines. Lines. 



Lines. 



. 3i 



M U 



41 



.. 41 







. 3 



U U 



4 



i-H 



1 1 



U 



