CLYPEASTER ROTUNDUS. 511 



specimen (23 mm ) all the prominent characters described in the young of C. 

 subdepressus were noticed, — the circular outline, short rosette, ambulacral 

 pores in the horizontal sutures above and below ; the odd ambulacrum was 

 also already somewhat the longest. 



Red Sea ; East India Islands ; New Caledonia. 



Clypeaster rotundus 



! Stolonoclypus rotundus A. Ao , 18G3, Bull. M. C. Z., I. p. 25. 



! Clypeaster rotundus A. Ac, 1872, Rev. Ech., Pt. I. p. 100. 



This species is distinguished from its Eastern representative, C. subde- 

 pressus, by its depressed test, more circular outline, and the spindle-shaped 

 outline of the ambulacral rosette, which has its greatest width nearer tbe 

 centre instead of nearer the extremity, as in the West India species. Tbe 

 rosette, in fact, is similar to the rosette of the young of C. subdepressus ; tbe 

 poriferous zone is broader ; the summit of the test more rounded. The edge 

 of the test is thin. The lower surface of the test is more flat, the mouth 

 not being sunk as deeply as in C. subdepressus. But the principal features 

 which prominently characterize this species are the small size of the primary 

 tubercles of the test, especially in the abactinal part of the test ; those on 

 the lower surface, though slightly larger, are not as large as the dorsal tuber- 

 cles of C. subdepressus ; apical system small ; genital openings small, placed 

 at angles of a smaller pentagon. Dried, the color is of an ashy-brown, which 

 in life is more violet. Owing to the tenuity of the edge of test the limestone 

 pillars extend more than half-way between the edge and the outer extremity 

 of the ambulacral rosette. 



The posterior pair of lateral ambulacra are somewhat longer than the 

 anterior pair. The odd ambulacrum is intermediate in length between the 

 two. The median ambulacral space, as well as the poriferous zone, is nar- 

 rower in the odd ambulacrum than in either of the lateral petals ; the lateral 

 posterior petals are usually more open than the others. 



Panama ; San Diego. 



* In the ambulacral system of Clypeastoids and Spatangoids the measurements of the petals alone 

 are given. 



