68 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



covered with very small, thin, imbricated, subequal plates ; many of 

 these plates bear a small spine which appears to be very easily lost. 

 These spines are elongated, slender and flattened, and they are termi- 

 nated sometimes by a conical point, and sometimes by two or even 

 three elongated and parallel points; their borders are sometimes 

 smooth, and sometimes furnished with one or two rather strong teeth 

 (pi. 92, fig. 5). They show rounded or oval perforations which are 

 unequal and arranged in a very irregular manner. The radial 

 shields are small, triangular, twice as long as broad, with a very 

 sharp proximal angle; the two shields of each pair are separated 

 from each other by an extremely broad space almost equaling the 

 width of the arm. 



In the interradial spaces the ventral surface of the disk is covered 

 with small plates bearing spines which become more and more reduced 

 towards the mouth shields. The genital slits are broad and very 

 evident. 



The mouth shields are of moderate dimensions, or even rather 

 small ; they are lozenge-shaped, and usually as long as broad with the 

 four sides subequal. The two proximal sides are straight and come to- 

 gether in a rather open angle ; the two other sides are gently concave 

 and are united by a distal truncated angle which sometimes forms a 

 small narrow and rounded lobe; the lateral angles are similarly 

 rounded. The two distal edges are thickened and rather prominently 

 raised, while the distal part of the shields which they delimit is 

 more or less strongly depressed, and forms a little hollow. In some 

 specimens the mouth shields, instead of being lozenge-shaped, are, 

 rather, triangular, and their distal border forms a more or less pro- 

 duced lobe, but the surface of this lobe is always very depressed, and 

 the sides are thickened and elevated. The adoral plates are large and 

 broad, and they broaden progressively outward from the median in- 

 terradial line; they give off outwardly a small process which sep- 

 arates the mouth shield from the first side arm plate. The oral 

 plates are triangular and rather high ; they are continued outwardly 

 as far as the first under arm plate by a plate triangular in form and 

 much elongated of which the base touches the first under arm plate 

 and which carries the outermost mouth papilla. The mouth papillae 

 are four on either side; the outermost papilla, which I have just 

 mentioned, is small and short, the following is large and thick, 

 lozenge-shaped, but much broadened at the base ; it is inserted both 

 on the oral plate and on the supplementary plate which follows it ; 

 the two others, conical in form, are rather narrow and not broadened 

 at the base. The terminal unpaired papilla is conical and short. 



The first upper arm plate is extremely short and very broad ana 

 bears on its distal edge a row of spines identical with those on the 

 dorsal surface of the disk; the second is larger, trapezoidal, very 



