74 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 
Gallery 
VIII. 
Table-ease 
28 . 
Wall-ease 
16 . 
Table-ease 
28 . 
Table-ease 
27 . 
All the ecliinoids thus far nientioned have a circular 
outline, with the mouth in the centre of the under surface, 
and the anus at the other pole suiTounded by an apical 
system of plates. Such echinoids are called Regular. In 
Bajocian time some urchins seem to have taken to moving 
generally in a single direction, and as one result the anus 
passed backwards from the apex, as may be seen in Pygaster. 
Then the mouth ]iassed forwards, as in Hijhodypeus, and this 
resulted in modifications of the front amlmlacrum and the 
hinder interambulacrum, e.g. Collyrites. At the same time 
in many forms the test became elongate, the jaws were lost, 
and the mouth developed instead a shovel-like lip, for the 
animal now took to burrowing through ooze and swallowing 
it on its way. Naturally the radioles became reduced to a 
coat of small, sometimes almost silky, spines. All such 
echinoids are called Irregular. 
The foreign Jurassic echinoids are placed on the floor of 
Wall-case 16. 
The British Cretaceous Echinoidea contain two distinct 
faunas — one from the Lower Greensand, and one from the 
Gault, Upper Greensand, and Chalk. The former is small, 
but the latter is the most interesting in the British series. 
Its most striking feature is the predominance of large 
specimens of Cidaris, of which a fine series of specimens 
from the Chalk is shown. One may note especially the 
example [E. 1952] of Cidaris sccplrifera with the apical 
plates, and those of Cidaris clavigcra [33,455 and :-(9,998] 
which show the jaws in position and the I’adioles attached. 
Following the Cidaridae come the Saleniidae, which have 
an additional plate in the apical system. The Diadematidae 
are represented by a large series of forms, of which Cyjdio- 
soma Kocnigi, from the Chalk, is the best known. The 
genera Glyphocypims and Zcuglopleurus are the forerunners 
of sea-urcliins with pitted tests, such as Temnopleuriis. The 
most interesting specimens are those of an Echinoihuria, a 
genus with imbricating plates, carrying further the type of 
structure begun in Pclanechimis and brought to a high 
development in Fhormosoma, Asthenosoma, and other genera 
now living in the abysses of the ocean. 
In the Irregular Echinoids of Cretaceous age the gradual 
change- of form and ornament that takes place in all groups 
of fossils as they pass up through the rocks has of late 
received careful study. By these mutations geologists are 
able to recoernise successive layers in the thick mass of 
