EDENTATA OF THE SANTA CRUZ BEDS. 



77 



This, the smallest known species, is 'much inferior to the exisiting 

 Zaedyus ciliatus in size. The carapace has no marked differences from 

 that of the other species, P. proximus, except that it is smaller, and in 

 most of the movable plates the two lateral longitudinal ridges are rather 

 more distinctly divided by transverse grooves into tubercles, but the dif- 

 ference is slight and perhaps not constant. A plate of the posterior mov- 

 able band measures 15x4 mm., and one of the first fixed row 8x6 mm. 



The dental formula is ^=T. In the upper jaw, - is very small, but the 

 successive teeth gradually become larger to the antepenultimate, the penul- 

 timate is somewhat smaller and the last tooth is quite small and of cylin- 

 drical form, not of oval section, as are most of the others. In the lower 

 jaw, T is very small, laterally compressed and almost laminar ; Y , 7 and T 

 are of similar form, but successively larger ; from -^ to ^ the teeth are much 

 thicker and have bevelled crowns ; 7 and are the largest of the series 

 and T?7 is quite small. 



The skull has a short, broad and low cranium and a slender, tapering 

 rostrum. The horizontal ramus of the mandible is very slender and in 

 the symphyseal region tapers to a comparatively sharp point, with rela- 

 tively long and slightly decurved edentulous portion ; there are two mental 

 foramina, each opening laterally ; the ascending ramus has a backward 

 inclination and the angular process is long and prominent. 



The following measurements are taken from a single individual (No. 



15,579)- 



MEASUREMENTS. 



Upper dentition, length 



Lower " " 



Skull, length on median basal line 



" extreme length 



Cranium, length to anterior rim of orbit. 

 Occiput, height 



" width at base 

 Palate, length 



Rostrum, width at anterior end 

 Mandible, length 



" of edentulous part 

 " depth below ^ 



thickness below ^ 

 " height of condyle 



Atlas, width 

 Axis, width of anterior face 



.Gig 'Axis and third cervical, length 009 



.027 Neck, length 029 



.055 First thoracic, length of centrum 004 



.066 " " width of anterior face ... .014 



.028 " " " " posterior face .. .012 



.0195 Lumbars, length of last three centra ... .021 



.030 Scapula, height 033 



.038 " greatest width 030 



.008 Humerus, length 038 



.049 " width of proximal end 012 



.006 " " " distal end 017 



.004 Ulna, length 041 



.003 " " of olecranon 015 



.0175 Radius, length 025 



.018 " width of proximal end 006 



.015 " " " distal end 007 



