PROCEEDIN-QS OF THE NATIONAL MUSEUM 



VOL. 7G 



believed that the information obtained from the specimen here de- 

 scribed will be of advantage in clearing up these points. 



The present specimen (Cat. No. 11944, U.S.N.M.) consists of a 

 carapace which is complete, except for the distal portions of the 

 fourth, fifth, sixth, and seventh costals of the left side and small por- 

 tions of the neurals. The carapace is broad, rovmded in front, and 

 somewhat truncate behind. At the free margins of the costals the 

 upper layers of bone project somewhat beyond the lower layers, caas- 

 ing a longitudinal grooving of the carapace; beyond this the margins 

 drop off gently to a thin edge. This was an exceedingly large turtle ; 

 the length of the carapace measured in a straight line is 735 mm., its 

 greatest width G40 mm., not including the extension of the ribs be- 

 yond the margin of the shell. It is composed of a nuchal plate, 7 

 neurals, and 8 pairs of costals, the 2 posterior pairs meeting in the 

 mid line. 



The nuchal measures 380 mm. across and 87 mm. in an antero- 

 posterior direction in the mid line, narrowing toward the outer ends. 

 It is 27 mm. thick in the central portions and tapers off to a thin edge 

 at the margins. Little ornamentation is apparent on this bone. 



The measurements of the costals of the right side (where all are 

 complete) are given in the following table : 



The ribs project beyond the margins of the carapace for some 

 distance; the best preserved one, that of the second costal of the 

 left side, extends out 92 mm. The lengths in the above table are 

 taken only to the border of the costal, not out onto the projecting 

 rib. Decided ridges are produced on the under sides of the costals 

 by the ribs, which are in most cases nearer the anterior borders 

 of the costals than the posterior borders. These ridges are plainly 

 seen on Plate 1, left, which gives a ventral view of the entire 

 specimen. 



The sculpture corresponds closely with Hay's description for the 

 typ)e fragments. It consists of ridges and grooves running at right 



