72 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery marked by five areas passing from near the anus to the 

 VIII. mouth, and these areas are fringed by the podia so that they 

 look like garden-paths or avenues (ambulacra). Thus it 

 appears that the ambulacral plates, those that constitute 

 these areas, are not the same structures as the tlooring-plates 

 of the groove in a star-fish. It would therefore in some 

 respects be simpler to compare a sea-urchin with an 

 Edrioastcr in which the covering-plates of the food-grooves 

 had become fixed, leaving passages for the podia, while the 

 flooring-plates had gradually been absorbed ; we must also 

 suppose the Eclrioaster to have turned upside-down, and its 

 ' anus and water-pore to have moved to the surface now 



uppermost. 



Table-case If now we examine the oldest British Silurian Echi- 

 noidea, namely, Echinocystis and Palaeocliscus from the 

 Lower Ludlow shales, we shall observe that the anus has 

 not yet reached the centre of the upper surface, that the 

 ambulacra have not met regularly around either that centre 

 or the anus, that in both genera the test was still flexible 

 with its plates neither fixed in number nor regularly 

 arranged, and that the pores for the podia are often between 

 the ambulacral plates instead of surrounded by them as in 

 later cchinoids. Moreover, it has been maintained that some 

 specimens of Palaeocliscus show traces of an inner set of 

 plates corresponding to the flooring-plates of the groove in 

 Edrioastcr. These genera, however, had, as our specimens 

 show, a well-formed jaw-apparatus of complicated structure, 

 only a little simpler than that found in a Mesozoic Cidaris 

 or in a recent Echinus. They must, therefore, have roved 

 actively in search of food. The movable spines (radicles) 

 borne by the plates are still small and not very different 

 from those of some Asteroidea and Edrioast^^roidea. 



Wall-case The Devonian rocks of Britain have yielded few remains 

 17a. of sea-urchins, but fossils from Germany (e.g. Lepidocentrus) 

 show that, while the test remained flexible, the plates in 

 each interradial area between the ambulacra were arranged 

 in columns, and that often each plate bore one radicle larger 



Table-case than the others. Li the British Carboniferous series are 

 29. genera, such as Falacechinus (Fig. 34, 1) and Melonites, that 

 still have the interambulacral plates in many columns ; but 

 in Archaeocidaris, or Ecliinocrinus, these plates are relatively 

 larger (Fig. 34, 2) and are definitely arranged in four 



WaU-case columns. In other respects Archacocidaris closely resembles 

 17a. the earlier forms of Cidaridae found in the Trias of the Tyrol 



