462 Mr. G. A. Boulenger on the 



suture with the eighth costal. The marginals are subequal 

 in length ; the third to tenth pair have a median socket for 

 the reception of the costal extremity ; the third to seventh 

 give attachment to the ligaments of the plastron. The 

 second, third, and fourth ribs are joined to two vertebras, the 

 rest to one only. The tenth or last dorsal rib presents this 

 peculiarity, that its extremity is connected, almost coalescent, 

 with the preceding rib, an arrangement which presents the 

 greatest similarity to that of the first and second dorsal ribs. 



Plastron. — The union of the plastron with the carapace is 

 by ligament, and remains so in specimens which have reached 

 full size. Digitations on the outer border of the hyo- and 

 hypoplastra, such as occur in Chelydra^ in which the plastron 

 is attached to the marginals by gomphosis, are absent, and 

 Platysternum would therefore enter Cope's group Lysosterna. 

 There are no fontanelles, and the plastron is altogether Emy- 

 dian. The bridge is formed to a slightly greater extent by 

 the hyoplastron than by the hypoplastron. The entoplastron 

 is moderately large, its length about two thirds the greatest 

 diameter of one of the epiplastra ; its outer face is oval, 

 broadest in front, its inner face more triangular, ending in a 

 short acute process. 



Skull. — The similarity of the skulls of Platysternum and 

 Macroclemmys is very striking — same hooked jaws, lateral 

 orbits, small frontals separated from the border of the orbits 

 by the pr^e- and postfrontals, well-marked ectopterygoid pro- 

 cesses, &c. The only important differences are that the orbits 

 are larger and the tympanic cavity smaller, the greatest 

 diameter of the latter being about half that of the former ; 

 that the supratemporal roof is still more developed, without, 

 however, attaining the stage of the marine turtles, in which, 

 as is well known, the parietal bones join the squamosals ; and, 

 lastly, that the jugal is completely enclosed between three 

 bones, viz. the postfrontal, the maxillary, and the quadrato- 

 jugal, an arrangement which does not occur in any other 

 Chelonian. The hyoid apparatus is largely developed. The 

 body consists of three ossified pieces, with a rhomboidal carti- 

 laginous space between ; there are two pairs of cornua, both 

 ossified. 



Cervical vertehrce. — The cervical vertebrae are in every 

 respect those of a typical Emydoid, not of a Chelydroid *. 

 The second and third are opisthoccelous, the fourth and eighth 

 biconvex, the fifth and sixth procoelous, and the seventh 

 biconcave; there are three ginglymoid articulations, v., vi., 

 vii., as in Emys orhicularis. 



Caudal v&rtebrce. — The three anterior are proccelous, the 

 * Cf. Vaillant, Ann. des Sci. Nat. ser. 0, x. art. 7 (1880). 



