MESOZOIC FAUNAS 181 



Fragmentary though they are, these ossicles illustrate 

 clearly the satisfactory palaeontological characters of 

 Echinoderms, in that zonal and morphological points 

 of great importance can be ascertained from them. 

 Astropecten in the Stonesfield Slate and Callovian, and 

 Metopaster in the Chalk, are Phanerozonate forms 

 sometimes found in an unbroken condition. 



Euechinoidea seem to date from the Trias. It is 

 unfortunate that Lower Mesozoic rocks in Britain are 

 ill-suited for preserving their remains. During Triassic 

 and Liassic times most of the orders of Echinoids were 

 differentiated, but they could not live in deserts and 

 shunned muddy water, so that our record of the class 

 was practically postponed until the Oolitic stage, by 

 which time most types were well established. 



The Cidaroida are undoubtedly the simplest, and 

 apparently the oldest, of Euechinoidea. They were 

 developed from Archaeocidaroid forms by reduction 

 of interambulacral columns to two, and loss of corona! 

 imbrication. Much of this change was achieved in 

 the Permian ; and Triassic Cidaroids, apart from vestiges 

 of flexibility, were much like those still living. This 

 morphogenetic stagnation of the order gives yet another 

 illustration of the persistence of primitive forms ; Lingula 

 and Nucula are the analogues of Cidaris in their re- 

 spective phyla. Cidaris (sens, lat.) occurs in the Lower 

 Lias and Rhaetic, but is common only in clear-water 

 deposits of Oolitic and Cretaceous age (PL xiii. fig. 3). 

 In the Inferior Oolite, Corallian and Chalk, com- 

 plete or fragmentary tests and radicles are often 

 abundant. The Diademoid (Centrechinoid) order seems 

 to have branched from the Cidaroid in Triassic times, 

 perhaps before full coronal rigidity had been attained. 

 The dominant characteristics of this group are pro- 

 gressive ambulacral complexity and multiplication of 



