74 GUIDE TO THE FOSSIL INVEETEBRATE ANIMALS. 



Gallery AH the echinoids thus far mentioned have a circular 

 VIII. outline, with the mouth in the centre of the under surface, 

 and the anus at the other pole surrounded 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 



Table-case passed backwards from the apex, as may be seen in Pygaster. 

 28. Then the mouth passed forwards, as in Hyhoclyjpeus, and this 

 resulted in modifications of the front ambulacrum and the 

 hinder interanibulacrum, 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 radicles became reduced to a 

 coat of small, sometimes almost silky, spines. All such 

 echinoids are called Irregular. 



Wall-ease The foreign Jurassic echinoids are placed on the floor of 

 16. 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 



Table-ease 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 sceptrifera with the apical 

 plates, and those of Cidaris clavigera (33,455 and 39,998) 

 which show the jaws in position and the radicles attached. 



Table-case 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 Cypho- 

 sonia Koenigi, from the Chalk, is the best known. The 

 genera Glyphocyplmis and Zeuglophurns are the forerunners 

 of sea-urchins with pitted tests, such as Temnopleurus. The 

 most interesting specimens are those of an Echinothuria, a 

 genus with imbricating plates, carrying further the type of 

 structure begun in PelanecMiiiLS and brought to a high 

 development in Phormosoma, 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 recognise successive layers in the thick mass of 



