148 CHELONURA TEMMIXCKII. 



plate; that of the fourth is most elevated; these prominences make on each side 

 a tuberculated lateral ridge, quite as high as the vertebral, so that the shell is 

 tricarinate. There are thirty-one marginal plates, of Avhich the nuchal, or 

 intermediate, is short, sub-quadrangular, and very extensive in the transverse 

 direction; it is concave anteriorly, and slightly so at its lateral borders, with a 

 prominence on its superior posterior face; the first marginal is irregularly penta- 

 gonal, with an elevated prominence at its outer and anterior part, at which begins 

 the lateral carina; the second is sub-trigonal, with its base before and rounded, 

 and its apex truncated and turned backwards; the third and fourth are elongated, 

 quadrilateral, with their anterior margins slightly convex; the fifth, sixth and 

 seventh are also quadrilateral, but are much more elongated and narrow; the 

 eighth is quadrilateral and broad; the ninth sub-rhomboidal; the tenth, eleventh 

 and twelfth are also sub-rhomboidal, each with a strong projecting point back- 

 wards, which gives the serrated and dentated appearance to the posterior margin 

 of the shell. On the sides of the shell, and between the lateral and marginal 

 plates, are interposed three supernumerary plates on each side. There seems, 

 however, to be some variety in their nmiiber, for Troost, whose accuracy no one 

 doubts, observed in his specimens four, whereas in the only specimen that I have 

 ever seen there were but three; the anterior large, pentagonal, with an acute angle 

 above, passing in between the lower margins of the first and second lateral plates, 

 and straight below, where it joins the fifth, and slightly the sixth marginal plates; 

 the second supplementary marginal plate is regularly quadrilateral, and is inter- 

 posed between the second lateral and sixth marginal, touching also slightly the 

 seventh; the third of these plates is also quadrilateral, and situated between the 

 second and third lateral and seventh marginal plates. 



The sternum is narrow and cruciform in shape, and resembles that of the last 

 described animal, though there the «ings descend a little from the sternmii, while 

 here they pass off at a right angle. Troost says it is covered with plates, similar 

 in form and number to those found on the sternum of the Chelonura serpentina; 

 yet in the specimen that I saw, the abdominal plates were subdivided, and it 



