42 TESTU0OPOLYPHEMUS. 



Description. The shell is remarkably depressed, nearly flat above, with its 

 margin entire, slightly revolute in front, and incurvated posteriorly. The vertebral 

 plates are five in number; the anterior is pentagonal, presenting an obtuse angle 

 forward; the remaining four are hexagonal, the posterior irregularly so. The first 

 lateral plate is irregularly triangular, with its basis rounded and joined to four 

 marginal plates; the second and third are pentagonal, with an acute angle above, 

 passing in between the vertebral plates; the posterior is irregularly quadrilateral, 

 the longest border directed downwards. The marginal plates are twenty-four in 

 number; the intermediate is irregularly quadrilateral, and largest behind, where it 

 joins the first vertebral plate; the supra-caudal is single, very large, having twice 

 the extent in the horizontal that it has in the vertical direction, its lower border 

 is incurvated, which gives this plate a remarkably bulging appearance. The first 

 margmal plate is pentagonal, the second square, the third irregularly quadrilateral, 

 and the fourth pentagonal; the four succeeding plates are quadrilateral, and of 

 greater elevation than breadth, slanting a little backwards; the ninth, tenth, and 

 eleventh marginal plates are irregularly quadrilateral; the tenth having its posterior 

 and superior angle truncated, where it joins the posterior vertebral plate. All these 

 plates are marked with concentric strise, which are most remarkable on the lateral 

 and marginal, and are often wanting on the vertebral plates; in many old indi- 

 viduals these disappear entirely, and leave the shell perfectly smooth. 



The sternum is thick and firm, prolonged beyond the shell in front, and deeply 

 emarginate behind. The gular plates are quadrilateral, and unite to form a spade- 

 like process, with its anterior extremity generally entire, but occasionally emargi- 

 nate; the brachial plates are quadrilateral, with their outer and anterior angles 

 rounded, the anterior border shortest and oblique in direction to receive the gular 

 plates; the thoracic plates are very irregularly pentagonal, narrow, and of great 

 extent in the transverse direction; the abdominal are quadrilateral and very large; 

 the femoral are also irregularly quadrilateral, with the longest border directed 

 forwards; the subcaudal plates represent oblong squares, and are most extensive 

 in the transverse direction. Of the supplemental plates, the axillary are oblong, 

 and the inguinal, semicircular in shape. 



