Report on the zoological collections made in the Indo-Pacific Ocean ...

;136 COLLECTIONS FBOM MELANESIA.4. OpMopinax stellatus. (Plate VIII. fig. C.)Pectinura stellata, Lyman, Trd. List, p. 3.Disk somewhat pentagonal, not pnfFed, with a central rosette ofplates varying a good deal in the extent to which it is well defined,and occupying about half the upper surface ; in some cases a centralplate and two not very regular circlets of variously sized plates ofirregular form can be made out. The interradial series of plates,which extend from the rosette to the edge of the disk, are stouterand more prominent than the radial series ; they are composed ofthree or four plates varying in size and form, and sometimes pairedat the margin of the disk ; the larger median have smaller plates oneither side. The radial series may, for its undivided portion, beformed by a single plate, but, as a more general rule, there arethree ; there are no smaller marginal plates. The bifurcated bandwhich embraces the base of the arm has usually three pieces oneither half. Just below the margin of the disk there is a prominentplate, which is placed interradially ; in shape this is irregularlycordiform ; above, on either side, it is bounded by a compressed ordiamond-shaped plate ; between it and the mouth-plate there is apair of smaller plates and several still smaller plates intervening ;on the side there is an elongated plate, with two smaller ones, lyingalong the edge of the genital slit. The radial shields are large andtriangular, and fill up all the space on the disk between the radialand interradial plates which is not occupied by the rosette.The inner mouth-shields have a straight outer edge, the sides areat first straight, and then bending inwards meet one another at amore or less acute angle ; they are longer than broad ; the spacebetween them and the edges of the mouth is rather loosely granularthe outer mouth-shields are about as broad as long, and have aslightly convex outer edge ; the umbo is well marked, and thatshield is not divided ; the side mouth-shields are well developed,and are separated from the genital slits by a small granulated patch.Seven mouth-papillte, the three innermost small, the innermostsmallest ; the next three large, increasing in size from within outwards; the outermost again small.The arms begin to narrow immediately after leaving the disk,and become very delicate at their free ends ; in a well-developedspecimen, in which the disk measures 17 millim. in diameter, theyare 70 millim. long, 5 millim. wide, and 4-5 millim. high at theirinsertion into the disk. Upper surface carinated ;upper arm-platesabout three times as wide as they are long, but, owing to the invasionof the side arm-plates, the aboral is longer than the adoral edgeboth these edges are straight.There is a pair of pores between the first and second lower armplates; the lower, like the upper, arm-plates have their aboral longerthan their adoral edges, and this, as on the upper surface, is due tothe encroachments of the side arm-plates, and is more stronglymarked near to, than far from, the disk. None of the edges of these