438 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Description. — The abactinal plates are much smaller and closer 

 together than in de'iisispina, the papular areas are smaller and have 

 but one papula, and there are fewer, shorter, and slightly stouter 

 spinelets, about 10 to 15 to each separate group and arranged in 2 

 or 3 series. These plates are frequently crescent-shaped and 

 sometimes tvro groups will be merged into one, partly surrounding 

 a papular area. In densispina there are actually much fewer plates, 

 which are larger and have a variable number of crowded spinelets — 

 usually 25 to 50. 



Along the lateral face of the ray is a rather indistinct line of su- 

 peromarginal plates, which are about twice as large as the adjacent 

 intermarginal plates, 2 to 3 rows of which form a triangular 

 area prolonged in a single series half the length of ray, beyond 

 which point the superomarginals are adjacent to the inferomar- 

 ginals. The latter are broader than long, perfectly distinct, 2 or 

 3 times larger than the superomarginals, and are separated from 

 the adambulacrals for three- fourths the length of ray by a series of 

 actinal intermediate plates; a second series extends nearly half the 

 length, a third about one-sixth, while 4 or 5 plates of a fourth 

 series are present close to the interradial line. 



On the furrow face of the adambulacral plates is a vertical series, or 

 sometimes a group of 3, occasionally 4, small spinelets. As a rule only 

 the uppermost is visible when the furrow is closed naturally. Sub- 

 ambulacral spines about 15 or 16 on proximal half of ray. Three 

 form a triangular group on the margin and are spatulate, sometimes 

 grooved at the end and much larger than the others. There are 

 usually 2 smaller spinelets behind these, while the remainder form 2 

 or 3 irregular series and are granuliform in appearance. 



Madreporic body situated at about middle of r fairly large, but 

 inconspicuous on account of the ridges being covered with spinelets ; 

 striae radiating. 



Type.— Cut. No. 37006, U.S.N.M. 



Type-locality. — Station 5518, Mindanao Sea, off Point Tagolo, 

 Mindanao, 200 fathoms, gray mud, globigerina, bottom temperature 

 54° F. ; 1 specimen. 



Remarks. — This species was taken in the general locality at which 

 H. densispina was collected. There is no way to decide whether it 

 is a variety of densispina, or a distinct form, on account of lack of 

 material. The differences in the spines and abactinal plates have 

 already been pointed out. In densispina the superomarginals are 

 much more conspicuous, and are in contact with the inferomarginals 

 for the length of the ray, there being only a fcAv intermarginals on 

 the disk. There is but 1 series of actinal intermediate plates, while 

 in microplax there are several at the base of ray. This causes the 

 inferomarginals to be bent outward so that they reach the interradial 



