146 NATURAL HISTORY BULLETIN 



papular area shows as a small convex or raised area, covered 

 with paxillffi rather lai'ger than those near by, and some of the 

 plates may bear a central spinelet larger than usual. There are 

 no larger central spines on the disk. The madreporic plate is 

 small with few gyri ; it is separated from the marginal plates by 

 four or five rows of very small plates. 



The jaws are relatively unusually wide and short, about as 

 wide as long, convex, and laterally broadly rounded. The mar- 

 ginal spines are longer than usual, slender, divergent, not 

 crowded, about six on the convex lateral margin, besides the 

 apical or peroral one, which is larger and longer than the others, 

 and acute. Each half of the jaw has about ten to twelve small 

 spinelets, irregularly disposed, the two groups separated by a 

 slightly prominent narrow carina, with no naked median sutural 

 area. 



The superomarginal plates have each a moderately long, slen- 

 der, terete, acute spine. They are about equally spaced and pretty 

 regularly graded, so that they form a very regular row. They have 

 no secondary spines at the base. The rest of the plate is finely 

 spinulose. 



The inferomarginals also bear a similar marginal spine, with- 

 out secondaries, their surface is covered with small, sharp, spaced 

 spinules becoming very small at the inner end. 



The adambulacral plates have a furrow-comb of seven or 

 eight slender spines, continued around the actinal edges by sev- 

 eral much smaller, slender, irregular spinules. The actinal 

 face bears one (or sometimes two) rather long, slender, acute 

 spines. About fifteen adambulacral plates correspond to ten 

 inferomarginal plates. 



The interactinal plates form a single chevron of about six spin- 

 ulose plates. Only one or two rudimentary pedieellariae are be- 

 ginning to form on some of the areas. On specimens of the same 

 lot, slightly smaller, there are only four interactinal plates and 

 no trace of pedieellariae. 



In form, size and general appearance this species resembles 

 P. mixtus. It can be readily distinguished by the dorsal plates, 

 which are rounded and convex, while in the latter they are flat 

 and closely tesselated, as well as larger; in this the jaws are 



