PLASTOMENID^:. 



I no and 1113, the specimens appearing on Cope's plate xxvi having the latter number. 

 Professor Cope states (Syst. Cat., p. 35) that 4 individuals were represented in his collection. 

 All these, if yet present, are included under the catalog numbers just given. The originals 

 of Cope's figs. I, 2, and 3, of his plate xxvi, were considered by him to constitute the type of 

 the species, and are said to belong to a single individual. Fig. I represents the outer end of 

 the left hypoplastron; fig. 2, apparently a fragment of the left hypoplastron at the bridge; 

 while Cope's fig. 3 is the distal end of a costal. In Cope's fig. 2 the left side represents the 

 sutural border for articulation with the hyoplastron, and the concave edge on the upper right- 

 hand side is the inguinal notch, which is here quite acute. If this identification of the bone is 

 correct, the notch was more abruptly formed than is usual in species of Trionychidae, and 

 quite as abruptly as in Plastomenus thomasi. In order to understand the relations of the bones 

 represented by the two figures, fig. 2 must be conceived as being removed to the left of fig. I 

 in such a way that the left edge, the sutural one, shall be turned 90 degrees and placed about 

 parallel with the upper border of fig. I . When the two figures are thus placed, there should be 

 perhaps about 35 mm. of space between their lower borders. Fig. 630 represents these bones 

 in the position suggested. 



The fragment of costal plate represented by Cope's fig. 3, plate xxvi, has a thickness ol 

 10 mm. The figure is reproduced (plate 86, fig. 2). The upper surface is beveled off some- 

 what and the upper layer of bone overhangs the deeper 

 layers. The upper surface of the bone is sculptured with 

 narrow ridges which run parallel with the free border of 

 the costal. Between these sharp ridges are furrows, 

 which are occasionally divided into pits by intersecting 

 ridges. There are 4 of these furrows in a distance of 

 10 mm. One fragment of the upper end of a costal is 

 found with the type. It shows the rib-head and was 

 FIG. 630.-Pl a ston,en U s l eftom itus. "ginally about 20 mm. wide The thickness at the 



Combination of two of Cope's fig- sutural border ls 6 mm - thru the nb ll ls 7 mm. The 



ures to show their relative positions. an g le between the articular border for the corresponding 



X^. No. 1113 U. S. N. M. Type, neural and that for the outer hinder angle of the neural 



next in front shows that the neurals were much wider 



behind than in front. The upper 25 mm. of the upper surface is covered with large irreg- 

 ular pits, about 3 in 10 mm. Distally they become smaller. It is possible that this fragment 

 does not belong to this species, but other fragments appear to form the transition to the 

 original of Cope's fig. 3, plate xxvi. 



The fragments which belonged to Cope's third specimen, and which furnisht figs. 27 to 31 

 of his plate xxv, belonged to a larger animal. It is possible, too, that they belonged to a 

 distinct species. As in the type, the costal is thickened distally and the sculpture consists of 

 ridges crossing the costal and connected at intervals by intersecting ridges. In the type of the 

 species the ridges are of nearly uniform size or perhaps they increase slightly in breadth and 

 distance apart toward the free margin of the costal. In the third specimen, however, they are 

 broader and more obtuse and they become distinctly narrower and more crowded toward 

 the margin, as is shown in Cope's figures. Furthermore, the sculptured layer of the type 

 overhangs the deeper layers, at the free margin, more than it does in the originals of Cope's 

 figs. 29-31. All these differences may, however, be due to variations in age and size or to 

 varying positions in the carapace. 



In the lot of bones representing this species there is a portion of a plastron, apparently a 

 part of the left hypoplastron from the inguinal region, and hence corresponding to the piece 

 represented by Cope's fig. 2, plate xxvi. This presents a sculpture in which the ridges run at 

 right angles with the hyohypoplastral suture. At this suture the thickness is 8 mm., but nearer 

 the free border it is 15 mm. An outer extremity of a hypoplastron in the lot shows a sculpture 

 different from that of the type individual, and more like that of Amyda ventricosa. 



More and much better materials are needed in order to determine the structure and 

 relationships of this tortoise. The form of the hypoplastron of the type and to some extent 

 the sculpture of the carapace seem to indicate that the species belongs to, or is near, the genus 

 Plastomenus. 



