514 MR MALCOLM LAURIE ON THE 



If, then, as I have tried to show, the genital operculum was a single structure, the 

 second free segment has still to be accounted for. Fig. 5 shows a portion of a specimen 

 in the Woodwardian Museum, in which a structure closely resembling the branchial 

 leaflets figured by Dr Woodward (pi. xix. figs. 3 and 4) can be seen through the 

 genital operculum at one side. I have never found any trace of such a structure on 

 detached genital plates, and am therefore inclined to consider that it was attached to the 

 soft skin of the body. Fig. 6 is a specimen from Dr Hunter's collection, which, I believe, 

 represents this portion of the body. One branchial leaflet is seen on the left and portions 

 of four on the right side of the figure. The structure connecting the two sets had to me 

 the appearance of a membrane, somewhat wrinkled and stretched. I ought perhaps to 

 state, that Professor Young, to whose kindness I am indebted for permission to describe 

 and figure this interesting specimen, differs entirely from me in his interpretation of it, 

 and considers it to be the inner surface of a limb, with the marks of muscle attachments. 

 These branchial leaflets are unsymmetrically cordate in form, being deeply cleft at what 

 was apparently their point of attachment. The margin is strengthened by a cord- 

 like thickening, and the surface covered by branching ridges, which radiate out from the 

 base of the cleft, and probably represent the course taken by the blood-vessels. 



The succeeding segments have well- developed sternites extending across the whole 

 width of the body, the posterior margin of each being strengthened by a broad border 

 exactly like that of the genital operculum. In the majority of specimens there is no 

 trace of appendages on these segments, but one or two specimens have yielded evidence 

 enough to make the structure and arrangement of these appendages fairly clear. The 

 specimen in the Woodwardian Museum (PI. I. fig. 5), which has been cited above as show- 

 ing branchial lamellae underlying the genital operculum, shows also the appendage of the 

 segment next behind the operculum, i.e., the third free segment. This is seen to consist 

 of a pair of plates resembling those of the genital operculum in general structure, but 

 differing from the genital operculum in the absence of a median lobe, and in the fact 

 that the plates overlap. each other in the middle line. A branchial lamella can be seen 

 apparently underlying the plate on the left side of the figure. In a specimen in the 

 British Museum (PI. II. fig. 8) the plate-like appendages are not shown, but traces of branchial 

 lamellae are visible on the fourth and sixth free segments. Finally, a specimen in the 

 Jermyn Street Museum (PI. II. fig. 7) shows, lying alongside the fifth free segment, the 

 remains of a plate-like appendage which shows traces of a branchial lamella attached to it. 

 The appendage shown in this last specimen agrees with the one shown in fig. 5 in 

 having a broad, thickened border. From its size it would have extended about one-third 

 across the segment, and from its unsymmetrical shape must have been one of a pair. 

 From a careful comparison of these specimens, and from slight traces on many others, I 

 have come to the conclusion that the abdominal appendages of Slimonia consisted of a 

 series of plate-like structures (PI. II. fig. 9), probably four in number, which were attached 

 to the anterior margins of their respective segments, and each of which bore on the side 

 next the body at least one, and probably more, branchial lamellae. These plates decreased 



