328 THE MEDALS OF CREATION. Cuap. IX. 
example, in which flint occupies the base of the shell, while — 
the upper surface is lined with crystals of calcareous spar. 
MICRASTER COR-ANGUINUM (Snake-heart). Lign. 107.—Of 
this genus there are many species in the Chalk. This type 
Lien. 107. MICRASTER (Spatangus) COR-ANGUINUM. 
Chalk. Lewes. 
Fig. 1.—View from above, showing the petaloid ambulacra. 
2.—View of the base, with the mouth. 
of Spatangidze are more or less oval, elongated, and heart- 
shaped, wider before than behind, with a sulcus, or furrow, 
in front. The shell is fragile, and composed of large 
polygonal plates ; the tubercles small and irregularly dis- 
tributed ; the spines are short. The mouth is transverse, 
situated anteriorly, and protected by a strong projection of 
the odd inter-ambulacrum, which is named the lip. The 
vent is terminal, and placed above the margin. There are 
but four ambulacra, and these are incomplete, comparatively 
of small extent, and situated in deep furrows. A large and 
new species of Micraster (Jf. cor-bovis, of Prof. E. Forbes), 
from the Sussex Chalk, is figured in Dixon’s Yossils, pl. xxiv. 
figs. 3, 4, p. 342. 
