72 GUIDE TD THE FOSSIL IN VEIITEBilATE ANIMALS. 
Gallery 
VIII. 
Table-case 
29. 
Wall-case 
I 7 a. 
Table-case) 
29. 
Wall-case 
17 a. 
marked by live area.s pa.ssiu«f from near the anus to the 
mouth, ami these areas are fringed by the podia so that they 
look like garden-paths or avenues (ambulacra). Tims it 
appears that the ambulacral plates, those that constitute 
these ai-eas, are not the same structures as the flooring-plates 
of the groove in a star- fish. It would therefore in some 
respects be simpler to compare a sea-urchin with an 
Udrioastcr in which the covering-i)lates 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 Edrioastcr to have turned upside-down, and its 
anus and water-pore to have moved to the surface now 
u])pennost. 
If now we examine the oldest British Silurian Echi- 
noidea, namely, Echinocydia and Palacodiscns 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 I’egularly 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 echinoids. Moreover, it has been maintained that some 
specimens of Falaeodiscus show traces of an inner set of 
plates corresponding to the flooring-plates of the groove in 
Edrioanter. These genera, however, had, as our specimens 
show, a well-formed jaw-apjjaratus of complicated structure, 
only a little sim])ler than that found in a IMesozoic Cidaiis 
or in a recent Echinus. They must, therefore, have roved 
actively in seai’ch of fo(jd. The movable spines (radioles) 
borne by the plates are still small and not very different 
from those of some Asteroidea and Edrioasteroidea. 
The Devonian rocks of Britain have yielded few remains 
of sea-urchins, but fossils from Germany (e.g. Leindocentrus) 
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 r.adiole larger 
than the others. In the British Carboniferous series are 
genera, such as Falaccchiwus (Fig. 34, /) and Melonites, that 
still have the interambulacral pl.ates in many columns ; but 
in Archaeocidctris, or Echinocrinus, these jilates are relatively 
larger (Fig. 34, 2) and are definitely arranged in four 
columns. In other respects Archacocidaris closely resembles 
the earlier forms of Cidaridae found in the Trias of the Tyrol 
