34 2 CHELONIA 



eleven or more shields, and there are four inframarginals : in 

 Staurotypti* xc/n'ni of Mexico the plastron is cruciform, with the 

 anterior lobe movable, covered with seven or more shields, accord- 

 ing to the fusion of the anal shields and the presence or absence 

 of the gulars ; there are only two inframarginals. The pubic and 

 ischiadic syrnphyses remain separate ; the temporal fossa remains 

 widely open, the postfrontals scarcely touching the parietals. 

 There are 23 marginal shields in Staurotypus, 25 in Dermatemys, 

 including the unpaired nuchal. The nuchal plate has a pair of 

 rili-like processes like those of the Chelydridae, but some of the 

 posterior costal plates, sometimes only one pair, meet in the 

 middle line, overlying or suppressing the corresponding neural 

 plates. The shell of these aquatic tortoises is rather flat, more 

 or less keeled, especially in young specimens, and in the fully 

 adult condition is about one foot in length. 



Fam. 3. Cinosternidae, represented by the single genus 

 Cinosternum, with about ten species in North and Central 

 America, and one in Guiana. Closely allied to the two previous 

 families, with which it agrees by the separation of the pubic and 

 ischiadic symphyses, the presence of an ento-plastral plate, the 

 possession of inframarginal shields (Fig. 61, 3, p. 315), the widely 

 open temporal fossae, and the rib-like pair of processes to the 

 nuchal plate. It agrees with the Dermatemydidae in the inter- 

 ruption of the neural plates by the meeting of several pairs of 

 the costal plates. There are 23 marginal shields ; five or four 

 shields, according to the presence or absence of the gular 011 the 

 plastron, and in some species these plastral shields become, with 

 age, more and more separated from each other by soft skin (see 

 Fig. 75). The shape and size of the plastron differ considerably 

 in the various species; in most of them, e.g. in C. pcnnsylvanicum 

 and C. leucostomum, but not in C. odoratum, the anterior and 

 posterior lobes are movable, with transverse soft hinges, so that 

 the animal can completely close its shell. The skin of the legs 

 and neck is so baggy and loose that these parts slip in, the skin 

 rolling off, when the creature withdraws into its shell. They 

 lay only a few from three to five elliptical eggs, which have a 

 shining, glazed, and thick, but very brittle shell. 



Cinosternum odoratum, the Mud-Turtle, or Stinkpot Terrapin, 

 so called on account of the disagreeable smell which exudes from 

 the inguinal glands. The head is disproportionately large, with 



