177 



The vertebral plates in the specimen, consisting of part of the second, tliini, 

 and itmrtli, are reversed coffin-shaped, and nearly twice as long as wide. 

 Their anterior border is convex, and the posterior border concave. 



The fifth vertebral plate is smaller than the preceding, and becomes eaiTier 

 narrowed from the sides toward the back end. 



The sixth plate is lozenge-shaped, about as long as it is wide, and occupies 

 the space between the truncated angles of the sixth and seventh costal plates. 

 The latter meet in the median line for more than half their width. 



The seventh vertebral plate is a very small lozenge-shaped bone, with a 

 crucial ridge on its surface, occupying an interval produced by the truncation 

 of the contiguous angles of the seventh and eighth pairs of costal plates. 



The costal plates, from the fourth to the sixth inclusive, are nearly of the 

 same width internally, and they successively become more widened out- 

 wardly. The seventh costal plate is rather wider at the extremities tlian 

 intermediately. The last costal plates are nearly as wide fore and aft as from 

 within outwardly. 



The surface of the carapace is sculptured tor the most part with broad, 

 rounded, and isolated concave pits resembling the impression of rain-drops on 

 a soft surface. Only near the outer border of the costal plates, where these 

 are preserved, do the pits become more or less confluent, usually in twos and 

 threes. The reticular ridges bounding the pits are broad and low, and often 

 as wide as the included pits. 



Measurements of the specimen are as follows : 



Lines. 



Length of tliird vertebral plate . . 23 



Width of third vei'tebral plate iu front 7 



Width of third vertebral plate behind 12 



Length of fourth vertebral ijlate 20J 



Width of fourth vertebral plate in front 7^ 



Width of fourth vertebral plate behind 10;^ 



Length of fifth vertebral plate , I'Ji 



Width of fifth vertebral plate in front 7 



Width of iifth vertebral plate at middle Sft 



Length of sixth vertebral plate - 10 



Width of sixth vertebral plate at anterior third 10 



Length of seventh vertebral plate 5| 



Width of seventh vertebral plate at middle 5 



Width of fourth costal plate fore and aft at inner part 23 



Width of fifth costal plate fore and aft at inner part 22 



Width of sixth co.stal plate fore and aft at inner part 21 



Width of seventh costal plate fore and aft at inner part IDJ 



23 G 



