REPORT ON THE DEEP-SEA MEDUSAE. 137 



connected with the overlying umbrella disk by the four arm pillars, and forms a thick 

 gelatinous disk of the base form of a quadrate prism, nearly half as thick as broad, 

 (40 mm. side length, by 16-18 mm. thick). If we transect two adjacent arm pillars at 

 their base and then turn back the half-loosened arm disk, its upper or aboral surface 

 which is turned towards the vestibule then becomes visible. It is quadratic and nearly 

 flat, only arched somewhat convexly towards the middle, whilst it slopes away very 

 gently outwards towards the lateral margins. The four perradial obtuse angles of the 

 quadrate pass into the distal ends of the oral pillars ; and the distal ends of the eight 

 limbs of the genitalia (fig. 6, sx) (which rise in pairs on the axial wall of the pillar canals, 

 cd) pass diverging, a little way on the oral surface of the arm disk. The lateral margins 

 of the pillars are slightly indented in the middle, and form at once both the lower 

 margins of the subgenital aperture (ig) and the distal margin of the four equilaterally trian- 

 gular, slightly convex subgenital valves (fig. 6, ivv) ; the latter are fused in some measure 

 at their lateral margins with the oral processes of the pillars, and so form the arm disk. 



The lower or oral surface of the oral disk is occupied by the frilled oral area (" area 

 orealis ") ; the suture of the oral cross and the eight-rayed rosette of tufts covering it 

 lies in the centre, whilst the eight arms run out round about it (fig. 7). The central 

 rosette of tufts is actually formed by four pairs of tufted frills, which correspond to the 

 four bifurcate branches of the four limbs of the oral suture, but the eight oral arms 

 which go out in pairs from the bifurcation of the distal end of the four oral pillars divide 

 at its bifurcated base so soon that they lie in the eight adradii almost from the first, and 

 the rosette of tufts also assumes a nearly regularly eight-rayed form, as on the oral axial 

 surface. The branched compacted bunch of tufts composing the rosette have the same 

 structure as the funnel frills at the distal part of the arms. If we cut off the tufts, we 

 see the regular suture of the oral cross (" sutura staurostomalis," fig. 1, as); as in all 

 Rhizostoma, it has arisen from the fusion of the frilled margins of the cruciform oral 

 opening, which is open at an early stage. 



The eight oral arms in our Leonura show, on the whole, the same peculiar formation 

 which was previously only known in Leptobrachia leptopus ( = Rhizostoma l&ptopus, 

 Chamisso, he. cit., pi. xxvii.). They appear as eight adradial, slender, band-shaped 

 appendages of the arm disk, whose length nearly equals the diameter of the umbrella or 

 only surpasses it a little. As in all Rhizostoruse multicrispse (Pilemidce and Crambessidce), 

 there are really two distinct principal parts on each arm, viz., the single-frilled upper arm 

 and the three-frilled lower arm (System, p. 582). In the Leptobrachidse, however, 

 the short upper arm (" epibrachium ") is quite rudimentary, and passed by concrescence 

 into the formation of the thick oral disk. The whole free part of the arm is therefore 

 formed here by the lower arm ("hypobrachium"). The proximal (upper) half of each 

 lower arm is naked, and without frills, and consists of a thin, triangularly prismatic 

 gelatinous band, whose three alleles run out into three narrow wings, each of which show 



(ZOOL. CHALL. EXP. PART XII. 1881.) M 18 



