SLIMONIA ACUMINATA. 



115 



There is good reason to believe that both these forms of opercular plates not only 

 belong to Slimonia, but to one species, namely, Slimonia acuminata. 



How, then, can we explain the diversity in the form of the median appendage in the 

 two opercula figured on Plate XVII ? 



If we refer again to its living analogue, Limulus, we shall find that in the Moluccan 

 and American king-crabs the same parts are modified in the males of both species, namely, 

 the antennas and the median articulated appendages of the thoracic plate or operculum 

 (see PI. IX and Woodcuts, I, II, fig. 34), corresponding to the coalesced median 

 appendage in Slimonia} 



m m 



I. II. 



Fig. 34. — Operculum or thoracic plate of Limulus poly phemus, Linn, (living). I. Male. II. Female, a, a. Anterior 



attached border : m = median appendage. 



At present we only know one form of antennas in Slimonia, but the discovery of the 

 two forms of thoracic plate (PI. XVII, fig. 1, 7 c, and fig. 2 ; and PI. XX, figs. 3 and 4) 

 affords strong grounds for assuming that we have here evidence of the male and female of 

 this fossil type. 



The Branchics (PI. XIX, figs. 3, 4). — In my account of Pterygotus bilobus, Salter, in 

 Part II of this Monograph (pp. 66—68, PL XI, fig. 2 ; PI. XII, figs. 1 a, 1 d; PI. XIII, 

 fig. 1 h) I have figured and described the branchial plates belonging to that species. 

 Similar leaf-like organs of a larger size have been obtained at Lesmahagow, associated 

 with Slimonia, some of which I figure on PI. XIX, figs. 3, 4, of the natural size. They 



1 This median lobe is even more readily identified with Limulus if we take Eurypterus instead of 

 Slimonia for comparison. In Eurypterus the division of the median lobe into two detached articulated 

 appendages remains clearly visible, whereas in the other fossil genera these are coalesced. 



This coalesced median appendage, with its broad lateral alse, forming together the operculum, is really 

 only the modified pair of appendages belonging to the first thoracic somite, the median pair of appendages 

 (or coalesced lobe, as the case may be) corresponding to the endopodites, and the external alae to the 

 exopodites of an ordinary pair of Crustacean limbs. 



This singularly modified pair of thoracic appendages perform the same office in Limulus as do the 

 antepenultimate thoracic legs in the female lobster, or the last thoracic limb on each side in the male; in 

 the former bearing the oviducts, in the latter the vas deferens. 



