386 CLARK : A NEW GENUS OF OPHIURAN3 



a right angle with each other; the sides are roundedly incised by the 

 proximal ends of the genital slits; the outer corners are broadly rounded; 

 and the distal border is concave or sometimes convex. The length of 

 the oral shields is equal to, or slightly exceeds, the breadth. 



The mouth papillae are five in number; the innermost is triangular 

 and sharp pointed, the others lower, truncated distally. Continuing 

 these is a series of five, more rarely six, papillae bordering the first 

 arm tentacle abradially, of which the outermost is broad and triangular, 

 with the apex over the proximal end, the others small, subequal, rounded 

 distally; opposite these on the first under arm plate are two, rarely 

 three, large tentacle scales, of which the outermost is about as large 

 as the outermost in the other series, and twice as large as the inner. 



The side mouth shields are narrow, about four times as long, as broad, 

 with parallel sides, and incised near their outer ends by the furrows 

 lodging the first arm tentacles. 



The first upper arm plate distal to the row of plates bordering the 

 radial shields is very narrow, almost bandlike, nearly spanning the 

 arm in dorsal view; the next is longer and wider, with converging sides 

 and a convex distal border; the following ones decrease regularly in 

 width, increasing in relative length, at the fifteenth or sixteenth becom- 

 ing triangular, twice as long as broad, with the distal border strongly 

 convex; beyond the sixteenth each upper arm plate is separated from 

 the preceding by an increasingly greater relative distance, as a result 

 of the increasingly broader union of the side arm plates. 



The arm spines are very short and slender, on the first ten side arm 

 plates three or four, commonly in two well spaced pairs, beyond these 

 five, toward the end of the arm four, usually in two spaced pairs, and 

 at the tip of the arm three. At first the arm spines are equal in length 

 and size; beyond the basal third of the arm the lowest increases in rela- 

 tive length, soon becoming twice as long as the others. 



The first under arm plate is fan-shaped, the outer border strongly 

 convex, the very short inner border strongly concave. The second is 

 usually slightly longer, twice as broad proximally, the lateral borders 

 slightly concave, the distal border with a median convexity and two 

 slight lateral concavities; the tentacles on either side are protected 

 by four scales inwardly and three outwardly. The third is similar, 

 but with a much narrower base and more converging sides. The fourth 

 is rhombic. The fifth is rhombic, shorter, in contact with the fourth. 

 The next two are shorter, rhombic, but with the outer angles cut away 

 by the tentacle grooves, widely separated from each other. The fol- 

 lowing ones have a low convex distal border and the outer part of 

 the proximal border cut away by the tentacle grooves. Beyond the 

 middle of the arms the under arm plates lie entirely between the ten- 

 tacle grooves; they become very minute in the distal portion of the arm. 

 Tijpe.— Cat. No. 38580, U. S. N. M., from Albatross Station 2785, 

 off the coast of Chile, in 449 fathoms. 



