TRIONYCHID^E. 



491 



The total length in the midline is 538 mm.; the greatest width, 665 mm. The greatest 

 height of the center of the carapace above the distal ends of the costals is only about 70 mm., 

 but in life the height may have been somewhat greater. The nuchal has an antero-posterior 

 extent of 75 mm., and an extent from side to side of 400 mm. There was probably a preneural 

 bone, but this region is damaged so that the presence of the bone is uncertain. If it was not 

 developt, the first neural must have been very large. Furthermore, there are some appearances 

 in the region in question which indicate that there was a preneural. The probable condition 

 in this part of the carapace is indicated in the text-figure (fig. 646). In the carapace, as restored, 

 there seems to be a suture between the inner anterior angle of the first left costal and a pre- 

 neural; but the supposed suture may be merely a fracture. There is also a fragment of bone 

 which appears to be a portion of the preneural and a portion of the first neural, but it is un- 

 certain whether or not the fragment belongs where it is placed. If present, the preneural was 

 probably wider at its anterior end than at the posterior. On the other hand, a very large 



first neural, like that of Amyda egregia, may 

 have occupied the entire space between the first 

 costals. The evident relationship of this species 

 to A spider etes coalescens Cope, as figured by 

 Lambe under the name Trionyx vagans, renders 

 the presence of the preneural more probable. 



Behind the preneural there were 7 neurals. 

 These had the usual coffin-shape, with the nar- 

 rower ends directed forward, except the sixth, 

 which is oval in form, and the seventh, which 

 has the broader end in front. The neurals 

 mostly decrease in size from the first to the last. 

 The table below gives the dimensions of the pre- 

 neural and the neurals. 



The first, seventh, and eighth costals are 

 considerably broader at their proximal, than at 

 their distal, ends; and the first is broadest of 

 all, its width near the neural being 88 mm. 

 Distally it is contracted to 52 mm. The next 

 three costals are about 60 mm. wide near the 

 neurals, the second and the fourth about no 

 mm. distally, and the third 87 mm. The sixth 

 and seventh costals measure about 50 mm. each 

 proximally, while distally their widths are respectively 108 mm., 133 mm., and 25 mm. The 

 eighth pair meet along the midline for a distance of 46 mm. while they form 230 mm. of the 

 hinder border of the carapace. The thickness of a costal at its sutural border and in the 

 middle of the length is 10 mm., while thru the rib the thickness is 14 mm. At their distal ends 

 the costals are beveled off to an edge. 



The ornamentation consists of abrupt and narrow ridges 

 which by their union inclose pits and furrows of irregular 

 size and form. There are usually 3 pits or furrows in a dis- 

 tance of 10 mm.; but in places there may be only 2, or again, 

 as many as 4. They are largest on the middle of the lengths 

 of the anterior and middle costals and over nearly the whole 

 surface of the hindermost costals; they are smallest along 

 the middle of the back and on the distal ends of the costals 

 of the anterior half of the shell. Toward the free borders of 

 the costals the pits diminish in size, and finally the beveled 

 border is left nearly smooth. In some places near the distal 

 ends of the costals (plate 88, fig. 8) the pits are nearly quadrate and in rows. 



On plate 88, fig. 7 shows the sculpture on the outer third of the second costal, the upper end 

 of the figure being the more anterior, the right side being toward the middle line of the 



FIG. 646. Aspideretes splendidus. Carapace of 

 type. X A. No. 3952 A. M. N. H. 



