22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



Although spatangoids breathe with petals, they arc able to live com- 

 pletely buried because they have fascicles that create currents, driving 

 air-laden water over the petals and a specially adapted anterior 

 ambulacrum with long tube- feet to maintain an open burrow to the 

 surface. Both these structures are lacking in the cassiduloids. 



Among the fossil cassiduloids, species of Hardouinia evidently 

 lived in a littoral environment. H. mortonis and H. porrectus are 

 found in great numbers in a sandstone at the top of the Ripley for- 

 mation. Dr. Norman V. Sohl, who has done extensive work on this 

 formation, states (personal communication) that the echinoids occur 

 with a typical heavy-shelled littoral molluscan assemblage including 

 Idonearca littlei (Gabb), Aphrodina sp., Pachycardium sp., Tellina 

 sp., OslYea cf. O. subspatidata Forbes, and Cyprimeria. According 

 to Dr. Sohl, this coarse sandstone appears from its texture, sedimen- 

 tary structures, and facies relationships to be a littoral beach deposit 

 formed in a high-energy environment. Littoral currents as well as 

 strong wave action probably agitated the sediments continuously dur- 

 ing the time of deposition. H. mortonis appears to be well adapted 

 to this environment. Its high test enabled the echinoid to live par- 

 tially buried in the sand with only its petals exposed. The covering 

 by sand of the nonpetaloid marginal portion of the test would serve 

 to anahor the echinoid and lessen the possibility of its being flipped 

 over by currents. The heavy, jutting bourrelets would aid in keeping 

 a passage open through the sand for the passing of food from the 

 phyllodal tube-feet to the mouth. 



SYSTEMATICS 



In order to avoid unnecessary duplication, I have not included for 

 any of the genera a synonymy of previous work or a history of 

 previous workers' opinions on the taxonomy of the suprageneric 

 taxa. This information is readily available in Mortensen's (1948) 

 great monograph. 



Order CASSIDULOIDA Claus, 1880 



Circular to elongate ; apical system tetrabasal or monobasal ; ambu- 

 lacra petaloid adapically ; periproct outside of apical system ; phyl- 

 lodes and usually bourrelets present; no jaws or gill slits in adult. 



Comparison with other orders. — The cassiduloids are distinguished 

 from the holectypoids by their usually elongate shape, petaloid ambu- 

 lacra, presence of phyllodes and usually bourrelets, and absence of 



