SPATANGOIDEA 559 



but the periproct is large and in contact with the apical system ; 

 tliese forms appeared in the Jurassic. The Collyritidae, also 

 Jurassic, had a marginal anus. The apical system was so much 

 elongated that two of tlie ocular plates are widely separated 

 from the other three, two opposite interambulacra meeting 

 between tlieni. Unmistakable Spatangoidea (Spatangidae and 

 Ananchytidae) appear in the Cretaceous, true Clypeastroidea 

 (Fibularites) in the Cretaceous, the other families in the Tertiary, 

 lieviewing these facts, we see that from the Holectypoidea we 

 can pass by insensible steps on the one hand into true Clypeas- 

 troidea, and on the other hand into true Spatangoidea. The 

 Holectypoidea differed from Endocyclica only in the position of 

 tlie anus, and the initial step in the backward shift of this organ 

 is seen in Pygaster. One result follows from this conclusion, 

 that the modification of the dorsal tube -feet into breathing 

 organs, and the consequent appearance of petals which accom- 

 pany the taking on of burrowing habits, were independently 

 developed in the Clypeastroidea and Spatangoidea, since these 

 features were absent in the more primitive members of both 

 groups. 



