EDENTATA OF THE SANTA CRUZ BEDS. 119 



ble *-, but there is a progressive increase in the size of the external lobes, 

 giving a more symmetrical shape to each succeeding tooth, though the 

 inner lobes are the larger in all of them ; the division of the postero- 

 external lobe into two pillars is most distinct in A , less so in 1 , and fre- 

 quently absent in s -. 



In the lower jaw T is of very variable size and is either of subcylindrical 

 or longitudinally elliptical shape ; ? is larger, but likewise variable, some- 

 times differing notably on the two sides of the same jaw, it is longitudi- 

 nally elliptical in shape and quite simple, or with a median vertical pillar 

 on the inner side ; ^ is much larger and obscurely trilobate on the outer 

 side, while ? is still larger and more distinctly lobate. The succeeding 

 teeth -5-, -5-, 7, , are trilobate, both internally and externally, or they may 

 be described as composed of three vertical prisms, without demarcating 

 grooves between the inner and outer anterior and posterior lobes, nor is 

 the postero-external lobe subdivided. 



In structure all the teeth are alike and are made up of three kinds of 

 tissue ; the external wall of the tooth is dense and shining, probably 

 of cement, with almost the appearance of enamel ; internal to this and 

 composing most of the substance of the tooth is a thick layer of soft den- 

 tine and in the middle of the grinding surface is a longitudinal ridge of 

 vaso-dentine, which sends out transverse branches into the lobes. All 

 the teeth are very high-crowned, giving great depth to the jaws. 



Comparing the dentition of Propalceohoplophorus with that of Glyptodon, 

 the most striking difference to be observed is the simplicity and small 

 size of the anterior teeth in the former. In the Pampean genus all of the 

 teeth except i are alike (and even i is but slightly less complex) and the 

 crowns have a more symmetrical shape ; each tooth is composed of three 

 rhomboidal prisms, of which the inner and outer lobes are of subequal 

 size, while the vertical grooves on the front and rear faces of the crown 

 are wanting; the postero-external lobe is not divided into two pillars. 

 Finally, in Glyptodon the ridges of vaso-dentine have a much more 

 complex pattern, the transverse branches giving off numerous ramifica- 

 tions, which are either feebly indicated or entirely absent in the Santa 

 Cruz genus. 



Skull (Plates XXIII, XXVII). The variability of the skull in this 

 genus is decided and individuals that agree closely in almost every other 

 detail of the exo- and endoskeleton often have skulls of quite different 



