ANATOMY OF OPHIOTHHIX VARIABILIS, ETC. 113 



raised, irregularly triangular surface (fig. 13), corresponding in 

 size to the fan-shaped muscular impression on the underpart of 

 the radial shield (figs. 9, 11). It is for the insertion of the adduc- 

 tor muscle (am). Orally to this muscular insertion the genital 

 plate narrows, is sharply rounded, and ends in a blunt point. A 

 transverse section of the genital plate made orally to the triangular 

 surface is not circular in outline, although the plate seems to be 

 cylindrical when seen from above ; it is more or less a bent curve 

 in outline (fig. 15), and the outer surface, which looks towards the 

 genital slit, is rounded and large, whilst the inner surface, which 

 is in contact with the side of the arm, is not so large and is concave. 

 Seen from below, the genital plate shows a furrow and the pro- 

 jection at the side of the raised part for the insertion of the 

 adductor, a rather narrow rod-like oral end, and a decided 

 enlargement aborally, the aboral edge being below the insertion 

 of the retractor (fig. 14). A side view shows foramina close to 

 the neck, the globose head, the projecting aboral ridge, the 

 nodule for articulation with the scale, and a foramen on the shaft 

 (fig. 15). 



The genital scale (fig. 13) is long, wide, irregularly triangular, 

 and boldly curved at its free interbrachial edge. The process 

 for the junction or, rather, articulation with the genital plate 

 is small, and has a slight concavity on its side towards the 

 genital plate's convex projection (or there may be an indefinite 

 nodule). The articulation is by an indefinite arrangement not 

 worthy of the name of ball-and-socket, but belonging to that 

 category. The free brachial edge of the genital scale, which 

 extends from the projection to the oral end of the scale, is 

 nearly straight, long, and thin. This edge bounds the genital slit 

 on the interbrachial side. There is some part of the edge of the 

 scale free, but most of the upper surface is covered by the derm 

 of the disk and is only seen after dissection. The specimen 

 which showed these details has a trace of a slip, apparently 

 muscular (fig. 13), which arises on the genital plate close to the 

 neck, and from the trochanter-like side projection close to the 

 head ; it passes between the raised surface for the adductor and 

 the projection for articulation with the genital scale and is in- 

 serted along the brachial edge of the scale just orally to the 

 projection for articulation. The slip is very thin, and crosses 

 over the outer end of the genital slit to reach the edge of the 

 genital scale. 



