20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 145 



CLYPEASTER PROSTRATUS (Ravenel) 



Plate 5, figures 2, 3; Plate 6, figures 1, 2; Plate 7, figures 1-4; text figures 12-17 



Scutella gibbosa Ravenel, 1845, Proc. Acad. Nat. Sci. Philadelphia, vol. 2, p. 253 



(not Scutella gibbosa Risso, 1825). 

 Clypeaster prostratus Ravenel. Mortensen, 1948, Monograph of the Echinoidea, 



vol. 4, pt. 2, p. 118, pi. 16, fig. 1 ; pi. 24, fig. 1 ; pi. 25, figs. 1, 2; pi. 26, fig. 5. 



(See this work for the pre-1948 references to this species.) 

 Clypeaster prostratus Ravenel. Cooke, 1959. U.S. Geol. Surv. Prof. Paper 321, 



p. 36. 



Before Mortensen (1948, p. 118) this species had never been ade- 

 quately described. Although Mortensen's description is thorough, it 

 is based on only two specimens. Since 38 specimens are now available, 

 a redescription is warranted. 



Diagnosis. — Species characterized by thin pentagonal test with 

 thick margin, flat area between end of petals and margin, and closed 

 paired petals. 



Material. — Thirty-eight specimens, all dried. 



Shape. — Smallest specimen 37 mm long, largest 91, average 70 

 mm; wide, average width 91 percent of length; length- width ratio 

 with little variation (text fig. 12) ; test pentagonal with truncated 

 posterior margin, pointed anterior with greatest width anterior to 

 center, in some specimens slight indentation at margin in inter- 

 ambulacra 4 and 1 ; margin thick, 6.5 mm thick in specimens 91 mm 

 long, area between margin and ends of petals very slightly sloping, 

 horizontal, or slightly depressed; petaloid area inflated; test low, 

 average height 17 percent of length (text fig. 13) ; adoral surface 

 flat. 



Apical system. — Central (pi. 7 fig. 4), small, madreporite large, 

 button shaped, five genital pores; ocular plates small. 



Ambulacra. — Petals broad, short, extending three-fifths distance 

 from apical system to margin; anterior petal (III) slightly longer 

 than others, in specimen 91 mm long anterior paired petals (II, IV) 

 shortest, approximately 9 percent shorter (25 mm long in holotype) 

 than petal III ; posterior paired petals intermediate (27.2 mm in 

 specimen 91 mm long), 6-8 percent shorter than petal III; paired 

 petals closed, anterior petal (III) open; interporiferous zone ap- 

 proximately twice width poriferous zone; number of pore-pairs 

 varying with size, in smallest specimen (37 mm long) 37 pore-pairs 

 in petal III, in largest (91 mm long) 61 ; as evident from text fig. 

 14, rate of addition of new pore-pairs decreasing in larger specimens ; 

 in specimen 91 mm long 59 pore-pairs in petals II or IV, 65 in petals 

 I or V. 



