422 Dr. A. Giinther on Hydromedusa. 



the Rio de la Plata, I have been mduced to examine the his- 

 tory of thisgenus, the species having lapsed into a singular state 

 of confusion from the time in which the genus was established 

 byWagler (Syst. Amphib. 1830, p. 135), who confounded 

 the species seen and figured by him with that which had been 

 so well described and ligured in Mikan's Delect. Flor. et Faun. 

 Bras. In this he was followed by Dum^ril and Bibron, 

 Gray, and other subsequent writers. 



Hydromedusa. 



Shell much depressed, with six scutes in the vertebral 

 series and twenty-four marginals. Sternum solid, the two 

 middle scutes directly forming a suture with the marginals, 

 without axillary or inguinal scutes. Median gular very 

 large, deeply penetrating between the postgulars. Neck 

 long ; head depressed, covered with skin, which, however, is 

 divided into numerous small scutes. Mouth of moderate 

 width ; jaws narrow, with a horny sheath. Eye of mode- 

 rate size. Limbs covered with granular skin, with a few 

 transverse scutes. Four claws only, in front and behind. 

 Tail very short. 



System of the Plate Eiver and the country intervening 

 between it and the Atlantic coast. 



The species known to me at present can be readily distin- 

 guished thus : — 



1. Hydromedusa Maximiliani (Mikan). 



Emys Maximiliani, Mikan, I. c. c. tab. 

 Clielodina flavilabris, Dum. & Bibr, p. 446. 



Chelomedusa Jlavilahris, Gray, Ann. & Mag. Nat. Hist.1873, xi. p. 304. 

 Hydromedusa Bankce, Giebel, Zeitschr. Ges. Naturw. 1860, xxvii. 

 tab. iv. 



Shell very flat ; dorsal scutes without tubercles at any age. 

 Nuchal comparatively small ; first vertebral large, with a 

 concave anterior margin, and intervening between the nuchal 

 and first costal. 



A small species which is fully adult with a shell six inches 

 in length, from the province of San Paulo (Brazil). Giebel's 

 statement that the type of H. Banhce came from the island of 

 Banka must rest upon some mistake. I have examined four 

 specimens, which show that the character taken from the 

 shape of the first vertebral scute is constant ; they fully 

 agree with Mikan's very good and Giebel's rude figure. 



