152 



E. C. CASE 



to the under surface of the plates as in the turtles (this point is some- 

 what uncertain), but in the posterior plates the ribs are separate. 

 The anterior plate is the broader and larger; the following ones 

 diminish rapidly in size, and do not reach nearly to the end of the 

 rib. It is evident that the plates did not cover the back, but lay on 



the sides of the body, with 

 a wide interspace over the 

 neural spines. In this as in 

 other respects the carapace 

 is much less perfect than 

 in the Otocoelidae. Cope 

 describes the shoulder girdle 

 of the Otocoelidae as lying 

 within the ribs. This does 

 not occur in the Diadectidae. 

 The plates lie between the 

 scapula and the ribs. So 

 peculiar is their position that 

 at first glimpse they have a 

 strong suggestion of begin- 

 ning plastral elements. 



The pelvis. — The pelvis 

 shows the same adaptation 

 to the depressed form of 

 body as the shoulder girdle. 

 The pubis and ischium are 

 flat and plate-like, and the 

 lower surface of the pelvis 

 formed by the four bones is horizontal. There is no trace of the 

 pubo-ischiadic suture, and the bones of the two sides meet in a 

 straight symphysis, which is marked by an elevated ridge. The 

 ilium stands vertically at a right angle to the other bones. (See 

 Figs. 2 and 18.) 



The ilium. — The crest of the ilium is nearly straight on the upper 

 edge, the anterior end is rounded, and the posterior end is continued 

 backward into a straight point. The lower portion forms the upper 

 part of the acetabulum. The anterior edge of the lower parts slants 

 forward and downward to join the pubis. 



Fig. 18. — Outline of the lower 

 surface of the pelvis of specimen No. 

 1075. One-half natural size. 



