108 



PROF. P. M. DUNCAN ON THE 



Mergui form, fig. 1, and it is Ophiothrician. The shape of the 

 genital plates and scales is slightly different. There is a peri- 

 stomial plate in Lyman's type, but not in O. variabilis. 



II. Mouth -frames, and Muscles ; Jaw-plates ; Teeth and Tooth- 

 papilla, and their Muscles. 



The aboral edge of the conjoined mouth-frames, seen from 

 above, is broad, and their union is by a stout, upward projecting, 

 long nodule (PL X. fig. 1). The edges of the mouth-frames 

 on either side of the mouth -slit are upwards projecting and 

 crescentic in their outline ; but the arch formed at the mouth- 

 slit is imperfect, there being a small outer indentation at the 

 median line which gives, with the outlines of the brachial edges 

 of the mouth-frames, a trefoil shape to the brachial extension of 

 the mouth. The incurving close to the junction of the mouth- 

 frames at the median line of the brachial region is small. 



The width of the brachial area of the region of the mouth is 

 limited by the comparatively radial direction of the jaws, and this 

 direction evidently has some connection with the size of the space 

 between the interbrachial edges of the mouth-frames in each 

 interbrachial space. These opposed edges are long from within 

 outwards, and slightly curved, and they are united by the fibres 

 of the interradialis aboralis muscle. The mouth-frames much 

 resemble those of Ophiophragmus as figured by Lyman in the 

 1 Challenger' Report, plate xl. fig. 4. 



It is evident that the distance between the interbrachial edges, 

 and therefore between adjacent mouth-frames, can be increased 

 or diminished by the action of the muscular fibres, and the dis- 

 tance does vary in the dead specimen. When the edges are far 

 apart, the obliquity of the jaws from the radial direction is greater 

 than when the edges are closer; when that is the case, the direction 

 of the jaws is more radial. The jaws are rather long, and there 

 is the usual well-marked depression for the vessels and nerves. 



It will be noticed in the description of the underpart of the 

 disk of the species (p.OO)that the side mouth-shields are not united 

 together orally to the mouth-shields, and that a skin intervenes ; 

 moreover t he separation alongthe median line of every jaw-an<j;lo 

 in considerable. Hence there is a space which extends from the 

 neighbourhood of I lie jaw-plate outwards between the side mouth- 

 ■hieldl 1w I ho adoral edge of the mouth-shield. The skin of this 



