466 CHELONIA. 



Spec. Chae. — Carapax elliptical or subelliptical, more or less convex, 

 with an obtuse vertebral bony ridge. Plastron subellipsoid, broader 

 anteriorly than posteriorly. Limbs scaly ; scales, on their anterior 

 aspect, large, not imbricated ; on the posterior aspect, moderate, 

 subtubercular. Nails rather stout. Tail superiorly keeled, subconi- 

 cal, and tapering. Generally olive-brown above, mottled with black ; 

 beneath yellowish, unicolor in the adult, abdominal region black in 

 the young. 



Syn.— ^mys marmorata, B. & G. in Proc. Acad. Nat. Sci. Philad. VI, 1852, 177. 



Emys nigra, Hallow, in Proc. Acad. Nat. Sci. Philad. VII, 1854, 91 ; &, in U. S. Pacif. 

 R. R. Expl. & Surv. VII, 1858, Kept. p. 3. PI. I.— Gray, Catal. Shield. Rept. Brit. 

 Mus. 1, 1855, 77. 



Actinemys marmorata, Agass. Contr. Nat. Hist. U. S. Amer. I, 1857, 444 ; &, II, 

 PI. in, figs. 5-8. 



Obseev. — When, on a former occasion, we described this species, it 

 was placed provisionally in the genus Dtut/s. A specimen of it having 

 been submitted to Prof Agassiz's insjDection, in anticipation of our own 

 final investigations, the result was the establishment of the genus Acti- 

 nernys, under which heading it is here recorded. 



It is much to our regret that we cannot enter, at present, into a 

 thorough study of its characters, as derived from adult specimens; 

 since those we possess are all immature, save two dried up carapaces, 

 one of which wants the head and hmbs, and the specimens belonging 

 to the Museum of the Smithsonian Institution are not available until 

 Prof Agassiz shall have completed his descriptions of the same. 



Desce. — The carapax, when viewed from above, is subelliptical in 

 its outline, narrower upon the thoracic region in the male (fig. 2) 

 than in the female (fig. 1). In the young, some are nearly circular, 

 whilst others exhibit already the peculiar subelliptical form which is 

 observed in the adult, only more regularly so than in the male. We 

 should not be surprised hearing that the subcircular ones are the 

 females, and the subelliptical, the males. As the latter grow up, the 

 abdominal or posterior half of the plastron dilates somewhat at the ex- 

 pense of the anterior or thoracic half The anterior margin is some- 

 what concave above the neck, and angularly emarginated posteriorly, 

 opposite the base of the tail. A bony and quite obtuse ridge may 

 be observed along the vertebral line, almost equally developed in both 



