112 



PKOF. P. M. DUNCAN ON THE 



upper surface of the arm. On turning the radial shield up and 

 separating it from the head of the genital plate, the following 

 structures are seen (PI. X. fig. 9). Close to the aboral edge is a 

 short curved, transverse space which gives attachment to a rather 

 broad, thin muscle (r), which passes downwards and is inserted 

 into a corresponding space on the aboral and upper end of the 

 genital plate (fig. 11, r). This extensor muscle passes aborally to a 

 downward projection of the radial shield, and also to the globose 

 head of the genital plate, and it is covered with thin skin (figs. 10, 

 11). Adorally to this space, on the under surface of the radial 

 shield, is a downward projection resembling a slightly flattened 

 hemisphere. Its lower, slightly flattened, and more or less curved 

 surface articulates with the globiform head of the genital plate 

 below (figs. 11, 13). Situated adorally to the projection on the 

 under surface of the radial shield, and separated from it by a 

 narrow transverse space, is a fan-shaped muscular attachment 

 (fig. 9, a?n), the arch of the space being placed orally, and the chord 

 of the arc being transverse and bounding the narrow transverse 

 space for the perihsemal canal adorally. The muscular marking 

 is large and the fibres pass downwards and slightly aborally, form 

 a stout little mass, and again expand and are inserted into an 

 expansion on the upper surface of a genital plate (figs. 9, 11, 13, am). 

 This adductor muscle is very distinct, and is evidently capable of 

 considerable extension during the contraction of the extensor, 

 and of corresponding contraction when in positive action. 



There is a very thin slip of fibres, which seems to be muscular, 

 passing from the outer edge of the radial shield below, and on a 

 line with the origin of the adductor, to the radial-shield side of 

 the broad genital scale (fig. 12, m). 



On removing the radial shield and looking downwards upon the 

 upper surface of the arm, the genital plate is seen with the scale 

 attached to the side remote from the arm. The genital plate is 

 moderately long, longer than the scale (gs), but much narrower 

 ( figs. 12-15). It has a head not very unlike that of a human thigh- 

 bone, continuous with a shaft by a broad neck, and at the interbra- 

 chial side of! lie neck is a nodular surface which has a little gibbosity 

 for the articulation of the genital scale (figs. 13-15). Aborally to 

 1 Ik: nodular pari, and extending beyond and below the head, is an 

 expansion which ends aborally in the ridge already noticed as 

 t iying insertion to the extensor muscle (figs. 11, 12, 15). Situated 

 orally to the Ik ad on the upper surface of the genital plate is a 



