CHELONIA. 27 



of the first three on the left side, and one of the coracoids (52) showing the rather 

 sudden and considerable expansion of its sternal or mesial half. 



The interval between the free extremities of most of the ribs, is about equal to 

 twice and a half the breadth of each extremity ; but the interval between the seventh 

 iph) and eighth {pls) rib, measured, like the others, at the terminal border of the 

 costal plates, is equal to thrice the breadth of the free part of the seventh rib. 



In this respect the Chelone planimenium resembles the Chel. my das more than it 

 does the Chelone caouanna, in which the interval between the free extremities of the 

 seventh and eighth ribs is less than that between the sixth and seventh. The length 

 of the costal plate of the fourth rib is twice that of the eighth rib, as in the Chelone 

 caouanna ; in Chel. mydas it is more than twice as long ; in Chel. imhricata it is only 

 one third longer. The marginal pieces in the Chelone planimentum seem to have been 

 narrow or slender in proportion to their length. 



The following admeasurements show that, in the large proportionate size of the 

 head, the Chelone planimentum corresponds with the existing turtles : 



Inches. Lines. 

 Length of the cranium ........ 5 6 



Depth of ditto 4 



Breadth of ditto 5 



Length of the carapace ........ 15 6 



Greatest breadth of ditto ....... 13 



Tab. IX and X satisfactorily illustrate the characteristic forms and proportions of 

 the unique specimen in the Cambridge Museum ; the carapace is figured of half the 

 natural size. 



Chelone crassicostata. Oioen. Tab. XI and XII. 



Testudo Plana. Konig. ' Icones Sectiles,' PI. XVI, fig. 192? 



That the extinct species of Eocene turtles attained larger dimensions than those 

 given above, is proved by a fossil skull from the Harwich clay, in the collection of 

 Professor Bell, which gives the following dimensions : 



Total length of the cranium .... 

 Its greatest breadth ..... 

 The antero-posterior extent of the symphysis menti 

 The vertical diameter of the orbit . 



do. do. of the nostri . , 



This skull differs from that of the Chelone planimentum in the minor depth of the 

 maxillary bone below the orbit (compare T. IX, fig. 3, with T. XI, fig. 2, 21), in the more 

 acute and attenuated muzzle ; but especially in the minor breadth and the difiFerent 

 configuration of the posterior margin of the symphysis of the lower jaw (compare T. IX, 



Inches. 



Lines 



8 







6 







3 







1 



9 







9 



