Ml BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



spines, not uniform as to size or arrangement. These are, when perfect, also some- 

 what grooved at the top and beset with unequal points. Some of the radial plates 

 have eight or nine Bubequal spines of the minor size, in a group, or in transverse lines, 

 and connecting plates also bear a few spines of secondary size. The dorsolateral 

 plates, however, usually have but one spine, or at best only one or two small acces- 

 sories. In the co type the abactinal spines are slightly stouter and rather less numer- 

 ous than in the type specimen. The maximum number on the radial plates is nine, 

 but is usually only live or six. It is only on plates where the spines are numerous 

 that there is any suggestion of the acervate or heaped arrangement characteristic of 

 acervata. Even this suggestion is not at all striking or conspicuous. Just above the 

 supermarginal plates there is a noticeable zone free from spines on the proximal half 

 of the ray. 



The superomarginal spines, proximally one to a plate, and separated for about 

 the length of the spines, form a regular lateral row, which rises to the level of the 

 abactinal surface at the interbrachial angle so that here the intermarginal zone widens 

 abruptly. The spines are stout, subcylindrical, either slightly tapered or slightly 

 clavate, blunt, about 3 mm. long, and a little weaker than the nearest dorsolateral 

 spines. There are about 50 superomarginal plates, and those near the end of the 

 lay usually bear- two or three stubby tubercles similar to the abactinal, while sporadi- 

 cally a proximal plate carries two spines. 



The inferomarginal spines are very similar to the superomarginals though a 

 shade longer, and a few of the proximal plates quite irregularly carry two or even 

 three spines; the dist almost plates usually bear but one. The intermarginal zone is 

 conspicuous and at the broadest part of the ray is about twice as wide as the length 

 of the superomarginal spines. Just below the inferomarginal spines which are on 

 the actinolatcral angle or curvature of the ray, is another narrower, spineless zone, 

 and then follow two series of actinal spines, somewhat stouter than the inferomarginals 

 the outer of which reaches the end of the arm, while the inner extends about three- 

 fourths the length, measured on side. 



The adambulacral plates follow closely after the inner actinals and are rather 

 crowded, there being about 33 to 10 inferomarginal plates at the base of the ray. 

 The normal arrangement is one and two spines to a plate alternating, the single spine 

 -landing in line with (he interval between the two of adjacent plates. The inner 

 spine of these alternate plates is slenderer than the others, tapered and is carried on 

 the angle between the actinal and furrow surfaces of the plate. The furrow spine is 

 2.8 to 3 mm. long, the others, which are untapered to very slightly so, a trifle flat- 

 tened, and blunt, are 3.2 to 3.8 mm. long. The first 10 or dozen plates beyond the 

 adoral carina of contiguous adambulacrals usually carry two spines each, after which 

 the regular arrangement becomes fixed. 



The mouth angle is formed of the mouth plates followed by four pairs of con- 

 tiguous adambulacrals, each of the latter carrying a single spine. This oral carina 

 is rather narrow, and its length about equals the width of actinostome. The entrance 

 to the furrow is narrow. The mouth plates bear each three spines, a somewhat 

 flattened truncate or round-tipped one directed over actinostome, a small one guard- 

 ing the mouth of furrow and carried on the inner furrow angle of plate, and a much 

 longer tapered suboral 3.5 to 4.5 mm. in length. (PI. 34, fig. 3.) 



