Feb., 1906. Carapace and Plastron of Basilemys Sinuosus. 253 



vertebral region. The nuchal shield is very small. It is narrow 

 in its superior aspect but rapidly widens as it rounds the anterior 

 margin. The first pair of marginals are long and narrow; the sec- 

 ond and third increase regularly in breadth. The fourth to seventh, 

 inclusive, overlie the bridge. Their distal articulation will be 

 described with the plastron. The eighth, ninth and tenth diminish 

 slightly in depth in the order named. The eleventh increases rapidly 

 in depth toward the mesial border. The pygal is deepest of the series 

 and is paired. 



The plastron is suggestive of great rigidity and strength as in- 

 dicated by the wide bridges with thickened buttresses and firmly 

 interlocking sutures, the massive anterior process formed by the 

 epiplastra, and the thickened and rugose borders. The pittings are 

 more deeply marked than on the carapace, and the arrangement of 

 shields is unusually complicated. In its entire length the median 

 sulcus crosses the median suture fourteen times. At no point does 

 it coincide with the median suture. Compared with the plastron 

 of B. variolosus as figured by Lambe* there is a great similarity. 

 However, the epiplastrals in this form are more produced, the ento- 

 plastron noticeably larger, the gular shields meet at the median line, 

 and the median sulcus is much more sinuous. Plate LXXVIII. 



The epiplastra unite at the median suture to form a thick and 

 rounded knob, which may be termed the gular eminence. The dis- 

 tal ends appear on the inferior surface as a pair of wing-like processes 

 which are directed postero-laterally. The entoplastron is sub- 

 hexagonal in outline and one-third broader than long. Its anterior 

 margin joins the epiplastra in two approximately straight lines which 

 meet in an angle of sixty degrees. The lateral borders are short, 

 the posterior angle very obtuse. The hypoplastra are somewhat 

 larger than the hyoplastra. In this specimen there is a break in their 

 connecting suture at the median line. The former articulate laterally 

 with peripherals three, four and five, and postero-laterally they pre- 

 sent a short articulating surface to peripherals six. The hyoplastra 

 articulate laterally with peripherals six and seven. The xiphiplastra 

 extend forward almost to the femoral notches. On the superior 

 surface the gular eminence terminates in a single short median ridge, 

 and is flanked by a pair of marginal ridges which extend half way 

 to the humeral notches. The entoplastron bears on a pair of low 

 converging ridges the facets for articulation with the precoracoids. 

 The xiphiplastra bear a similar but larger pair of facets for ligamentary 



•Geol. Surv., Canada. Vol. III. part II, page 39. 



