40 



The plastron is flat except at the sides where it beuds evenly upward, the lobes are 

 short and broad, and the sternal bridge long. The entoplastral is roughly pentagonal 

 and rather broad. The epiplastrals are of not unnsual size and shape, whilst the hyo- 

 plastrals are relatively large. A divided intergular shield separates two small gulars, 

 behind which are well developed humeral shields. The pectorals narrow rapidly 

 toward the sides where they and the abdominals meet a series of inframarginals that 

 overlap the peripheral bones. All the sulci are deep and conspicuous except those 

 marking the position of the inframarginals, the anterior boundaries of the gulars, and the 

 division of the intergular. These latter, however,, are sharply and clearly defined. The 

 sutural line between the hypoplastrals and the xiphiplastrals is shown in the smaller of 

 the two fragments. 



The sculpture consists, when most rugose, of well excavated pits of a rounded hex- 

 agonal outline arranged quincuucially ; the dividing ridges are angular and narrower 

 than the pits are wide, their angularity and height being more pronounced at the junc- 

 tion of every three pits with each other. The formation thus, of three-surfaced, pyrami- 

 dal projections between the excavations, gives to the shell of this turtle its very charac- 

 teristic and rugose appearance. There are about seven pits in a space of 20 mm. In 

 places a number of pits, as many as half a dozen or more, may coalesce so as to form a 

 continuous groove. The sculpture is most rugose near the edges of both carapace and 

 plastron, elsewhere its roughness is modified and generally toward the centre of the shell 

 the surface is comparatively smooth. 



On the inner or upper side of the plastron (fig. 5, page 39) the rugose sculpture 

 extends inward for some distance from the free edges of the lobes, more particularly at 

 the extreme anterior end, where also the bone is very much thickened. A decided 

 thickening also occurs in the axillary region. The oval outlines on the xiphiplastrals (P, 

 fig. 5 ) show the position of smooth, slightly raised, flat surfaces that are apparently facets 

 for the articulation of the pubic bones. 



Large fragments of the carapace show that the cosials are thin and are united to the 

 comparatively thick marginal bones by suture and that the sulci are deep and well 

 defined. In some of the marginal bones the rib-prolongations from the adjacent costals 

 are preserved. The rib-heads of the costals are apparently well developed. 



In 1901, the anterior half of a plastron (fig. 6, page 41), with the front border of the 

 carapace intact, was obtained below the mouth of Berry creek. This specimen, repre- 

 senting an individual, with a plastron about "840 mm. in length, gives the relative 

 position of the plastron to the carapace anteriorly and also shows an enlargement of the 

 right pectoral shield to the left of the median line of the plastron resulting in a decided 

 diminution in the size of the left pectoral shield. 



Additional material includes two or three neural bones, some of the bones of the 

 endoskeleton and a few small, thick, conical scutes from the tail. These latter have 

 an upper surface whose sculpture is an exaggeration of that of the most rugose parts of 

 the shell. 



The types of Compsevxyx og-mius, Cope, from the '' Fort Union group" of Milk river, 

 consisting of two small, poorly preserved fragments of costal bones, show sculpture 

 markings so similar to those characteristic of A. variolosus, that the writer is of the opinion 



