Fep.,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. vartolosus as figured by Lambe* theré 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 LX XVIII. 
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. 
