36 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



of which the base occupies the whole surface of the plate, and the 

 height is a little greater than the breadth. These tubercles are ar- 

 ranged one behind the other in a single, though very irregular, row, 

 extending over two-thirds of the length of the radial ribs; they 

 become more numerous at the ends of the ribs, where they form 

 a little group ; they pass onto the lateral faces of the ribs on either 

 side of the base of the arms, but they do not reach the ventral sur- 

 face. In the interradial spaces the border of the disk shows a row 

 of similar tubercles extending from the extremity of one radial rib 

 to that of the other. In the radial spaces at the bases of the arms 

 there are a few additional tuberosities, but these are smaller and 

 only two or three in number in each space. The other plates of the 

 dorsal surface of the disk are quite unarmed. 



The lateral surfaces of the disk included between the much-swollen 

 distal ends of the radial ribs are narrow and very strongly depressed 

 into a sort of oval pit on the borders of which are the genital slits, 

 which are rather short and narrow, and arranged parallel to each 

 other. The ventral border of the interradial depression forms a 

 somewhat projecting edge on which are some conical tubercles, iden- 

 tical with those of the radial ribs, which on the sides pass into the 

 arm spines. 



The ventral surface of the disk is covered with small polygonal 

 plates which are unequal, very close together, not imbricated, but in- 

 stead very exactly in contact, forming a true pavement. These plates 

 are continued onto the ventral surface of the arms, becoming there a 

 little larger and somewhat thicker. Further, there appear toward the 

 base of the arms and on certain of them small short conical tubercles, 

 with rounded tips, very much smaller than the tubercles of the disk, 

 which are continued in varying numbers onto about twenty of the 

 arm segments; these little tubercles disappear with the progressive 

 tapering of the arms. 



The outlines of the mouth plates are quite invisible, for these ele- 

 ments are hidden beneath a covering of small plates identical with 

 those which cover the rest of the ventral surface. The mouth angles 

 carry a bundle of dental spines which are very close together, elon- 

 gated, flattened, and rounded at the tip ; the ventral surface of these 

 spines shows a very evident groove throughout their whole length. 

 The sides of the mouth angles are simply furnished with very small 

 granules passing gradually into the neighboring plates, which are 

 entirely flattened; the ventral surface of the mouth angles, behind 

 the bundle of dental papillae, likewise shows small granules which 

 become obliterated and flattened little by little as the distance from 

 the papillae increases, and which similarly pass over into the plates 

 of the ventral surface of the disk. 



