22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I45 



Periproct. — Inframarginal, located near posterior margin; on holo- 

 type 3.5 mm from margin, opening irregular in shape, usually elon- 

 gated transversely. 



Peristome. — Central to slightly posterior, pentagonal, pointed an- 

 teriorly, truncated posteriorly. 



90 1 



Hso 



|?o 



3 60 



< 



Z50 

 in 

 |40 



Q. 

 1 

 LU 



§30 



Q. 



°20-| 



1 10- 



+ ++ + + -H- 4 + l+ 



+ 

 ++ 



"15 3) 30 40 50 60 70 80 90 100 

 LENGTH IN MILLIMETERS 



Fig. 14. — Clypeaster prostratus (Ravenel). Number of pore-pairs in ambula- 

 crum III relative to length of test. 



Adoral plate arrangement. — Primordial interambulacral plates 

 much smaller than ambulacral plates, difficult to see because of exten- 

 sion of ambulacral plates over suture, on inside of test (text fig. 16) 

 interambulacral plates only visible near outer edge of basicoronal 

 plates, on outside of test (text fig. 16) plates more exposed, extending 

 almost to peristome ; basicoronal interambulacral plates separated 

 from postbasicoronal plates by two pairs of ambulacral plates (text 

 fig. 15) ; 6 to 7 ambulacral, 4 or 5 interambulacral postbasicoronal 

 plates in each series on adoral surface. 



