4i6 SCIENCE PROGRESS 



in the Mesozoic as in the Palaeozoic Crinoids ; and they are 

 obviously undergoing reduction in the Mesozoic forms. 



Turning to the Echinoids, it is evident that the Palaeozoic 

 forms represent a plastic and fluctuating stage of transition 

 in the evolution of the phylum. The inconstancy in the number 

 of the rows of plates of the corona, and the flexibihty (in most 

 cases) and variety of form of the test, with its fragile, thin, 

 and loosely articulating plates, which occur in the earliest 

 Echinoids in comparison to the Mesozoic genera, are character- 

 istics which present an analogy to the manifold variety of forms 

 exhibited by most groups near their point of origin, just as 

 we have already (p. 399) seen to be the case in the Mesozoic 

 Bennettitecey just before a reduction to a working optimum had 

 been arrived at by the early Angiosperms. The Palaeozoic 

 Echinoids displayed a tentative variability in various divergent 

 directions, and the Cidaroids alone out of all these offshoots 

 from the parent stock survived to perpetuate the race. From 

 the CiDAROiDEA all the Mesozoic and modern Echinoids can 

 be derived. 



Although the Ordovician Bothriocidaris is at present the 

 oldest known Echinoid, it is probable that its single row of 

 plates in each interambulacrum and its rigid test represent 

 an early experiment in reduction. On the other hand, in 

 view of the multiple and inconstant number of rows in the 

 interambulacral areas in other Palaeozoic Echinoids such as 

 Archaeocidaris, Melonechinus (Melonites), Palaeechinus, etc., 

 the latter state of things should presumably be regarded as 

 the more primitive feature. This is all the more probable, 

 seeing that in post-Palaeozoic forms the reduction to the 

 optimum of two rows in each area has become a constant 

 characteristic.^ 



Among the earliest forms a fairly near approach to the 

 Cystids seems to be made by EcJiiriocystis ponium of the Upper 

 Silurian, in which the flexible test is composed of a mosaic 

 of numerous (10 at the diameter) irregularly arranged and 

 undifferentiated interradial plates with small spines, each 

 ambulacral area containing four rows of pore-plates united at 

 the upper pole without forming an apical disc; the anus is 



' The Cretaceous Tetracidaris has indeed four rows of plates to each inter- 

 ambulacrum but they are reduced to two rows at the apical region. 



