SLIMONIA ACUMINATA. 



117 



double row of branchiae supported on two similar but more membranous plates, they might 

 easily have lain concealed beneath the outer operculum. 



This latter point seems established on the evidence of specimens both in the Museum 

 of Practical Geology and in the British Museum, showing two (or three?) opercular plates 

 of the same shape associated together, and evidently belonging to the same individual. 1 



Fig. 36.— View of the inner side of an opercular plate of Slimonia acuminata, from Lesmahagow, Lanarkshire, in which 

 the two reproductive openings are seen near the centre of the upper attached border (a, a) of the plate. 



Two of these plates, moreover, exhibit two small rounded prominences, which, there 

 can be little doubt, were ovarian openings. (See Woodcuts, figs. 35 and 36.) 



Fig. 37. — Thoracic plate or operculum of Slimonia sp. ?, giving indication of the presence of branchigerous 



plates concealed beneath, a, a. Attached border. /,/. Free border, m. Median appendage. 



This paucity in the number of pairs of appendages to the body-rings is a truly larval 

 character, reminding one of the condition of the larval Decapod (see Part I, Plate IX, 



1 See 'Quart. Journ. Geol. Soc.,' 1867, vol. xxiii, p. 31. H. Woodward, "On the Structure of the 

 Xiphosuea and their Relationship with the Eurypterid^:." 



