REPOET ON THE OPHIUROIDEA. 29 



plates. The mouth frames are low and depressed with great sockets for the first 

 mouth tentacles. There is nothing but a thin lime crust to represent the peristomial 

 piece. The slender, cylindrical genital plate is so long as to reach the mouth shields, 

 and is broken in several pieces; for a quarter of its outer length there is united to it the 

 genital scale, which then spreads out like a thin ^^^ng. The arm bones within the disk 

 are long and cylindrical, instead of short and discoidal. Just outside the disk they are 

 still more elongated with a median contraction ; and they present a complex arrangement 

 of processes and hollows. (See PI. XXXVIII. figs. 1-5.) 



Ophioplinthus medusa, Lym. (PI. XXIV. figs. 7-9; PI. XXXVIII. figs. 1-5). 



OpMoplinthus medusa, Lym., Biili Mus. Comp. Zool., vol. v., part 7, p. 105, pi. ii. figs. 30, 31, 

 40, 1878. 



Scaling of lower interbrachial space coarse (four or five scales in a transverse row). No 

 scales on arm tentacles. Usually three very minute, peg-like arm spines, equally spaced. 



(Type specimen from Station 156.) Diameter of disk 16 mm. Width of arm 2 mm. 

 Mouth papillae short, square, closely soldered, four or five on each side, with one, 

 diamond shaped, at apex of the jaw. Mouth shields rounded triangular, with an 

 angle inward, sides curved, and often a re-entering curve without; they are frequently 

 furrowed or cracked. Side mouth shields irregularly rhomboidal, sometimes l)roader 

 within, where they touch. First under arm plate rounded triangular, with a 

 blunt angle inward, and a long curved outer edge; second plate five-sided, with 

 an obtuse angle within, and a curved outer edge; beyond this they are broader than 

 long, with a blunt angle within and a wavy curve without. Side arm plates 

 thick and smooth, meeting broadly below and touching above beyond the sixth plate. 

 Upper arm plates rounded diamond shaped, with a longer angle mthin ; they 

 often are cracked or grooved. Disk high, covered with thin, irregular, overlapping 

 plates; the central primary one about 2 mm. in diameter and conspicuously the largest, 

 and in each l3rachial space is a primary plate larger than the rest. Kadial shields 

 irregular, four sided, with rounded corners, broader on the outer edge, separated their 

 entire length hj a Inroad wedge of smaU scales. On the under surface of the disk are 

 numerous irregular plates between the genital scales, which are thick, with a wavy out- 

 line. The genital opening runs less than half-way to the margin. Three very short, 

 blunt arm spines, situated low on the outer edge of the side arm plate. A pair of small 

 tentacle pores, each surrounded by a raised burr, situated near the inner angle of the 

 second under arm plate, on its edge; and a similar but minute pair on the third plate. 

 Beyond this the pores are difficult to see and lie near the base of the under arm spine. 

 The second paii" of mouth tentacles protrude from an oval rim, having on each side a 

 couple of ill-defined tentacle scales. Colour in alcohol, gTcy. 



