CHAP. X. | CONTINUITY OF THE CHALK. 489 
characteristic and harmoniously varying series are 
of censiderable value in the discrimination of the 
different formations. In the soft white chalk of the 
south of England their remains are extremely abun- 
dant. Perhaps the most abundant and characteristic 
fossils in the chalk are the Cidaridze, and these more 
than any other chalk fossils illustrate the peculiar 
conditions under which the chalk has been laid down. 
The great spines of Cidaris are attached to the plates 
of the shell by a central ligament which passes from 
the cup on the spine to a perforation in the ball on 
the plate, and by a membrane which rises from the 
plate and passes over the base of the spine. The 
spines are, however, so disproportionately large, and 
the soft matter softens and decomposes so rapidly 
after death, that it is difficult to keep the spines 
attached to a specimen prepared even with consider- 
able care. In the chalk, tests of Cidaris are fre- 
quently preserved absolutely entire, with all the 
spines in position ; so that by carefully working out 
the chalk with a penknife, we can here have the 
whole animal perfect. It is difficult to see precisely 
how this result can have been produced. The urchin 
must have sunk into the soft chalk-mud and 
been covered up by a sufficient quantity to support 
its spines and test, and allow the whole to become 
gradually compacted into a solid mass. One of the 
new deep-sea Cidarites belongs to a genus which 
had previously been supposed to be extinct, but the 
chalk-mud forms generally do not show any special 
approach to any particular chalk species. Still the 
general character of the group is the same. The 
Echinothuridz were previously known only as chalk 
