UPPER, OR FLINTY CHALK. 185 



4. Five pentagonal {scapular) plates, filling up the angles in the 

 superior margin of those last described, each having a semilunar de- 

 pression on their marginal edge : these form the margin of the pelvis 

 properly so called. 



These sixteen plates are succeeded by 



5. rive semilunar ossiculae {clavicles), attached to the articulating de- 

 pressions of the scapular plates. 



6. Five cuneiform ossiculae {cuneiform or humeral hones,) attached to 

 the clavicles. These are the first bones of the arm, and their superior edge 

 is divided into two articulations, from which the tentaculae are sent off. 



7. Numerous reniform ossiculee, by which the aperture of the pelvis is 

 closed. 



The plates of the pelvis are convex, sometimes umbonated in the 

 centre, and ornamented with radiated ridges on the external surface. 

 Their markings vary in different examples, and even in the same indi- 

 vidual ; specimens occurring in which some of the plates are nearly smooth' 

 and others richly ornamented. 



Fig. IS. Tab. xvi. represents a plate, with prominent ridges extend- 

 ing from the centre to the angles, the intermediate surface being covered 

 with numerous diverging striae. In fig. 14, the centre is slightly um- 

 bonated, and the markings are coarser and less regular than in the former 

 variety ; the ridges are obtuse, and the diverging striae few and irregular. 



The plates in fig. 15 are perfectly smooth, with the exception of a few 

 folds near the margin. 



In every instance, however, the edges of the plates are more or less 

 crenulated, and when united form a suture in the same manner as the 

 scales of the tortoise, but they readily separate when the chalk is removed. 

 The central or abdominal plate is larger, and more depressed, than the 

 surrounding costals; the latter are readily distinguished by their j^en^a- 

 gonal, and the intercostals by their hexagonal form. ' The scapula are ge- 

 nerally less ornamented than the rest of the series, and are easily identified 

 by the semilunar cavity in their upper edge ; this articulating surface is 



