28 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



long. These shields are armed with short, rather thick, conical 

 spines, which are especially developed in their distal portions. These 

 are larger than the spines of the rest of the dorsal surface of the 

 disk, into which, however, they grade imperceptibly. 



The ventral surface of the disk is covered by an integument which 

 is darker and thicker than that on the dorsal surface, and this is 

 studded with little rounded plates, which are extremely reduced and 

 widely separated from each other. Toward the mouth shields these 

 plates become a little better developed, and in their immediate 

 vicinity they move closer together and even touch each other. The 

 genital slits are broad and very long. The two genital plates, inter- 

 nal and external, which are short, are limited to the distal extremi- 

 ties of these slits, and project outward somewhat. 



The outlines of the mouth plates are more or less obscured by the 

 integument. Thanks to the entire absence of pigment they may, 

 however, be made out through it. The mouth shields are small, tri- 

 angular, and a little broader than long. The rather large adoral 

 plates are thick, triangular, narrower within than without, and in 

 contact in the median interradial line; they are a little longer than 

 broad. The oral plates are triangular and very high, in contact 

 throughout their entire length. Together with the tooth papillae 

 the mouth papillae form a little bundle of short, slender, and spinu- 

 lose spines continuous with the teeth which are fairly regular in their 

 arrangement and which have exactly the same form as these papillae ; 

 beside this bundle of terminal papillae two distinct and independent 

 lateral mouth papillae can be recognized. 



The upper arm plates are divided into two lateral halves each 

 having the form of a triangle of which the base lies along the median 

 line, and separated from its fellow by a narrow interval ; the sides of 

 this triangle are slightly projecting; these plates are broadly sepa- 

 rated from each other. 



The first under arm plate is quadrangular, a little broader than 

 long. The following plates are elongated, a little longer than broad, 

 rectangular, with the proximal border narrower than the distal, 

 which is notched in the middle; they are all in contact with each 

 other. Beyond the disk the plates become smaller, triangular, with 

 a truncated proximal angle, and are separated by soft tissue. 



The side arm plates, which are only slightly projecting, bear four 

 and sometimes five spines at the base of the arms ; these spines are 

 very delicate and cylindrical, provided with very strong denticula- 

 tions on the proximal border, and smaller and more crowded ones 

 on the distal border ; they are all shorter than the segment and sub- 

 equal, though sometimes the dorsal spine is a little larger than the 

 others. The first ventral spine bears very strong teeth on its proxi- 



