CHAP, xj CONTINUITY OF THE CHALK. 489 



characteristic and harmoniously varying series are 

 of considerable 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 

 fossUs in the chalk are the Cidaridse, 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 diflB.cult 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 Oidarites 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 

 Echinothuridse were previously known only as chalk 



