68 Proceedings of the Biological Society of Washington. 



pentagon produced into rather long slender arms, which are as long as 

 the distance from their bases to the center of the disk. 



The abactinal surface is covered with more or less regular hexagonal 

 plates which are largest in the central portion, within a circle the periphery 

 of which is indicated by the madreporite. From the apical plate a regu- 

 lar row of rune plates, all approximately equal in size and smaller than 

 the plates in the center of the abactinal region, runs to the arm bases, 

 where it terminates against two or three large, irregular, wedge-shaped 

 plates. In the outer two-thirds of the abactinal area this radial row is 

 bordered with a similar row on either side. In the interradial triangles 

 the plates decrease regularly in size from the center to the margin, re- 

 maining hexagonal as far as the row adjoining the superomarginals, in 

 which the plates are very irregular in shape and in size. The plates of 

 the abactinal surface are naked, except for a peripheral row of flattened 

 granules, which on the four or five central plates in the radial rows is 

 supplemented by a second similar row of higher granules just within it; 

 the most proximal of these plates bearing the second row of granules 

 usually has it developed only along the distal margin, while the plates of 

 the two rows adjoining the median row have the second row developed 

 on the adradial side. A few plates, especially in the central region, have 

 from one to three widely scattered granules on their surface. 



The madreporite is prominent, rounded triangular, with a very convex 

 and irregular surface. 



The superomarginals are similar and of equal size as far as the arm 

 bases, thence decreasing rather rapidly to the arm tips. They are some- 

 what tumid, especially those of the arms, and are naked except for a 

 bordering series of closely packed flattened granules, which disappears 

 on the outer half of the arm. Those in the interbrachial arc may bear 

 scattered granules on the surface, and may have in addition one or two 

 extra rows along the lower border and extending for a short distance 

 along the sides. 



The inferomarginals are everywhere much lower than the supero- 

 marginals, though reaching the same vertical plane ; in the interbrachial 

 arc they correspond to the superomarginals, but on the arms they do not 

 decrease so rapidly in length, and so come to overlap the bases of the 

 next succeeding superomarginals. 



On the actinal surface the decrease in size of the inferomarginals at the 

 arm bases is much more abrupt than in the case of the superomarginals, 

 and along the arms the former are much narrower, being twice as long 

 as broad, whereas the superomarginals are always broader than long. 



The inferomarginals are bordered with a row of flattened granules like 

 the superomarginals, while those of the interbrachial arc have an addi- 

 tional row actinally extending part way up the sides, and just beneath 

 the upper margin numerous closely packed granules which extend down- 

 ward along the sides. 



The actinal intermediate plates do not extend beyond the third infero- 

 marginal; that is, they are entirely confined to the triangle between the 

 adambulacrals and the inferomarginals of the interbrachial arc. They 



