304 BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



Toward euil of arms primary meshes become smaller ami the distinction is not so 



evident. 



On basal third of ray, adjacent to adamhulacral plates, is a series of fairly 

 reo-ular plates slightly larger than other plates of general perisome, having twenty 

 to thirty spinelets, but this series soon becomes irregular. One or two tiers of 

 plates above this series there is an indication of a very iiTegular marginal series, but 

 it does not extend beyond basal fourth of ray. Meshwork on actinal and lateral 

 surfaces much closer than on dorsal, the secondary plates pla^-ing a very subsidiary 

 part in the make-up of skeleton. Frequently two or three or even more papula- 

 occur in the papular spaces. 



Adamhulacral plates about tmce as wide as long, separated by a distinct and 

 conspicuous interval or suture. Armature dense, consisting of (1) two small saber- 

 shaped spinelets placed one above the other high in furrow, except on first six to 

 eight plates, which have a vertical series of tlu-ee to five. Occasionally on distal 

 portion of ray there is but one furrow spinelet, but two is the usual number. (2) On 

 actinal surface of plate are thirty-five to forty slender pointed spinelets arranged in 

 three or four transverse series on inner half of plates and too crowded to be regular 

 on outer half, decreasing rapidly in length and caliber from the furrow outward, 

 the outer spinelets being sharper than the inner. 



Mouth plates forming a salient angle, closing actinostome, the sutures between 

 first adambulacrals and mouth plates marldng off a regular pentagon inclosing the 

 ten mouth plates. Of this pentagon the prominent interradial dental sutures mark 

 the radii, while the closed ambulacral furrows form equally regular interradii. 

 Armature very dense, the six or seven marginal spinelets being somewhat enlarged, 

 much flattened, and with blunt or truncate tips; two or three small spinelets form 

 a longitudinal series on furrow face of the plates. The mouth plates are larger and 

 more conspicuous than in H. fohiacantha. (See PI. 78, fig. 3.) 



Madreporic body prominent, situated midway between center of disk and 

 interbrachial angle, the surface being covered with tufts of small spinelets. 

 Color in alcohol, whitish. 



Anatomical notes. — The appearance of a mesh witliin a coarser mesh presented 

 by an exterior view of the plates is due partly to the presence of the spinelets wliich 

 hide the slight gaps between the small secondary plates, although many of the latter 

 are firmly united into a ridge as the figure will show. Undoubtedly the plates are at 

 first independent grains and later coalesce. Comparison of the figures of H. 

 polyacantha and 77. clnrH will demonstrate the difference in the skeleton structure. 

 77. clarM has no sign of the regular arrangement of the papulse, characteristic (so far 

 as known) of polijacantlia. (Compare PI. 112, fig. 3, wath PI. Ill, fig. 26.) 



Variations. — The specimens from stations 2923 and 2935 are placed with tliis 

 species not without serious misgivings. The lack of several specimens from the type- 

 locality by which to determine the range of individual variation has led me to place 

 the specimens in question \vith clarJci, although at first sight they look hke a very 

 (Ufferent form. The abactinal spinelets are fewer, and more spaced as regards the 

 small clusters. The meshes are very irregular, but in the example from 2935, which 

 is large (R = 89 mm., r=12 mm.), the characteristic mesh within a mesh formation 



