590 NORTH AMERICAN INDEX FOSSILS. 



many rows of tubercles. Mouth opening ten-angled. Anal open- 

 ing large, pear-shaped, situated between the mouth and the pos- 

 terior edge of the test. Apical disk small, central. Jurassic- 

 Cretacic. 



24. H. planatus Roemer. (Fig. 1922.) Comanchic. 

 Subcircular, subconical, flattened on under surface. Ambulacra 



with six irregular rows of tubercles. Pores in single pairs. Inter- 

 ambulacra with numerous narrow plates, each plate with a nearly 

 horizontal row of small tubercles. 



Widely distributed in the Fredericksburg and Washita of Texas. ' 



25. H. charltoni Cragin. Comanchic. 

 Larger than preceding and less elevated ; quinquelaterally rotund ; 



apex tending to rise slightly from a somewhat flattened summit- 

 region ; periproct two or three times smaller in proportion than in 

 preceding, inframarginal, and widely separated from peristome; 

 subovate. 



Upper Washita of Texas. 



Order CLYPEASTROIDA Duncan. 

 XVII. DiPLOTHECANTHus Duncan. 

 More or less pentagonal echinoidea, generally of large size, 

 with central peristome and small marginal periproct; dorsal or 

 abactinal surface convex, high; ambulacra petaloid; ventral or 

 actinal surface hollowed to the deeply-placed central peristome; 

 actinal furrows straight; internal structures in the form of pillars, 

 investing the ambulacra with a double wall, but not forming con- 

 centric partitions near the edges; surface with fine spines. Ter- 

 tiary-Recent. 



26. D. reticulatus (Linnaeus). (Clypeaster (Echinanthus) rosa- 

 ceous A. Agassiz.) Tertiary-Recent. 

 Subelliptical, with rounded posterior end, the form obscurely 



pentagonal; posterior petals longest; actinal surface often flat for 

 a short distance before descending to deep peristome. 



Oligocenic ( ?) of Cedar Keys, Florida; Tertiary of West Indies 

 and San Domingo; living to depth of five fathoms off coast of 

 South Carolina to the Bahamas, Cuba and Guadaloupe. (There 

 seems to be no important difference between the Tertiary and 

 Recent forms.) 



