114 



PKOF. P. M. DUNCAN ON THE 



It is perfectly evident that the genital plate may have its 

 position altered by the arm being depressed or elevated as much 

 as is possible, and that the plate may be parallel with the arm, or 

 may form a wide angle with it. The genital plate being on the 

 side, and over the arm, may have the adductor relaxed so as to 

 allow the radial plate and the dome of the disk to rise. It 

 appears that under these circumstances the relative positions 

 of the plate and scale alter, and that there must be a con- 

 siderable movement of the scale on the pivot of the brachial 

 projection. 



A few small overlapping scales are placed on the interradial 

 edge of the genital slits between the oral termination of the 

 genital scale and a process which is in contact with the aboral 

 edge of the mouth-shield. These processes appear, on a super- 

 ficial examination, to be parts of the mouth-shields passing 

 outwards in the interradial spaces, and limiting, orally, the 

 derm of the underpart of the disk. Lyman has shown that they 

 are really only closely attached to the mouth-shields with con- 

 nective tissue, and maceration separates them. Each of these 

 processes is double and each touches its fellow at the median 

 line ; they are broadly attached to the aboral edge of the mouth- 

 shield and are collar-shaped. At the median line aborally their 

 edges diverge, sloping outward and then towards the arm, and 

 reaching the genital slit. The derm is attached to the edges of 

 these processes, which are properly genital scales, instead of to 

 the aboral edge of the mouth-shields. 



IV. The Structure of the Arm-bones. 



These are formed generally after the type of the Ophiothricidae, 

 as described and figured in Lyman's ' Challenger' lieport on the 

 Ophiuroidea (pi. xlii. figs. 5-8). 



There arc, however, some points of difference. On the aboral 

 surface of an arm-bone (PI. X. fig. 16) the cavity for the recep- 

 1 ion of the umbo of the opposed bone is large, and below it is a 

 prominent ridge which passes downwards, in the median line, to 

 the peg. The projection in the median line may be narrow 

 or considerably swollen ; and in either case the peg at the 

 lower end is broad, projects aborally, and has a transparent, 

 long rather narrow articulating surface on each Hank (PL X. 

 figs. L6, 17 ; PI. XI. figs. 21,22). The surfaces project so much as 

 to give the appearance of two lateral pegs ; but that is not the case, 



