Feb., 1906. Carapace and Plastron of Basilemys Sinuosus. 251 



posteriorly and lies opposite the suture between pleurals vn and 

 viii. A ninth irregularly rounded plate intercolated between the 

 last pair of costals may be regarded as a post-neural. 



The pleural plates extend only about two-thirds of the distance 

 from the neural suture to the margin of the carapace. Their articu- 

 lation with the neurals has been described in the preceding para- 

 graph. Distally they articulate with the marginals, breaking joints 

 in a mosaic pattern. Pleurals 1 to 111 inclusive are directed obliquely 

 forward. Pleural iv stands at a right angle to the median line. 

 From this point the remainder of the series becomes more and more 

 directed obliquely backward until the last pair conforms to the wide 

 A-shape of the first pygal. Pleural 1 is much the broadest of the 

 series, articulating anteriorly with the nuchal and peripherals one 

 and two. Distally it abuts the whole mesial border of peripheral 

 three and a small anterior portion of four. Pleural 11 is long and 

 narrow, meeting only two-thirds of peripheral four. Pleural in is 

 widest at the distal end, articulating antero-laterally with the fourth 

 and laterally with the fifth peripheral. Pleural iv is uniform in 

 width, and has a distal articulation similar to the perceding. Pleural 

 v is slightly expanded toward the extremity and presents an almost 

 equal surface to marginals six and seven. The suture between Pleu- 

 rals v and vi is unusually sinuous. The latter element is noticeably 

 narrower and has its distal articulation similar to the preceding. 

 Pleural vn is shorter, and much narrower at the mesial end. It 

 articulates distally with peripheral eight by a short suture, but chiefly 

 with nine. Pleural vni is shortest and smallest of the series. It 

 arises from the eighth and ninth neurals, and is more uniform in 

 width than the preceding. It meets peripheral nine in a short 

 antero-lateral suture as well as half the mesial surface of peripheral 

 ten. 



The first pygal plate has the form of a wide A. Its lateral wings 

 are co-extensive with the last pair of pleurals, with which they articu- 

 late anteriorly. Laterally they abut a third of the mesial surface 

 of peripheral ten. The posterior surface is concave to receive the 

 rounded surface of the succeeding element. The second pygal is a 

 wide irregular figure, convex on the anterior surface and bounded 

 by three concave facets posteriorly. These facets articulate with 

 the eleventh pair of peripherals and the third pygal. Laterally this 

 bone also presents a small pair of surfaces to the postero -lateral faces 

 of the tenth pair of peripherals. The third pygal is not more 

 than half as large as the second and is sub-hexagonal in outline. 



