EDENTATA OF THE SANTA CRUZ BEDS. 41 



different parts of the same carapace that isolated plates are often very de- 

 ceptive. 



Exoskeleton. - - The head-shield is still incompletely known and the 

 only specimen of it that I have seen consists of a number of plates at- 

 tached to the skull of P. robustus ; these are of irregularly polygonal 

 shape, with more or less rounded angles, of very different sizes, and with 

 coarsely pitted surface. 



The carapace, as in other Santa Cruz armadillos, forms no scapular 

 buckler and the pelvic shield appears to be relatively small ; one specimen 

 displays eight rows of fixed plates in this buckler, but almost certainly 

 lacks one or more of the hindermost bands. All the scutes are thick and 

 massive, and are conspicuously sculptured, in patterns that differ more or 

 less strongly in the different species. In the movable bands the scutes 

 of the anterior and median regions are long and narrow, and somewhat 

 more than one third of each is overlapped by the preceding plate ; pos- 

 teriorly and on the sides of the carapace the plates become considerably 

 broader in proportion to their length. The marginal plates are shaped 

 much as in the movable bands of Dasypus, with curved free borders and 

 overlapping each other but little ; they are not sculptured, but quite 

 smooth and finely punctate. The fixed plates of the pelvic buckler are 

 very thick, shorter and broader than the movable scutes and with a sculp- 

 tural pattern which is a modification of that seen in the latter. 



Dentition. --In all of the species the dental formula is -TO. In at least 

 two of the species and probably in all, - is implanted in the premaxillary ; 

 - is larger and laterally compressed and has a bevelled masticating sur- 

 face ; ^ is still larger and of similar shape. All the succeeding upper 

 teeth have flat grinding surfaces and are, except the last one, more or less 

 distinctly bilobate. The teeth of the two sides converge anteriorly to - 

 and then become parallel. In the lower jaw the first four teeth are small 

 and compressed, -5 is much larger, but of similar shape, while the posterior 

 five teeth have a rudely triangular grinding surface. In both jaws the 

 large posterior teeth have the dentine very clearly arranged in three 

 layers ; the periphery of the tooth is formed by a dense, smooth, enamel- 

 like layer and a similar tissue forms the core ; while between the two is a 

 layer of softer dentine, which wears away more rapidly, leaving the harder 

 parts in relief. A similar arrangement is found in the Pampean genus 

 Rtitatus. Still another pecularity of these teeth, also shared by those of 



