Vol. 51.] ANNIVERSARY ADDRESS OP THE PRESIDENT. lxxiii 



" At this stage," says Packard, " the young swim briskly up and 

 down, skimming about on their backs by napping their gills, not 

 bending their bodies." This locomotion of the young Limulus, by 

 swimming upon its back, near the surface of the water (by means of 

 its gill-feet), agrees very closely with the habit of Apus, of Cliiro- 

 ceplialus, and Artemia, and is extremely suggestive of its affinity 

 to the phyllopoda, with which, at this stage of its existence, it has 

 many points in common, as well as with the trilobita. 



It is interesting to notice that the Xiphosdra (king-crabs) — 

 which form the surviving representatives of this ancient order of 

 the Merostomata, and are so widely distributed in the Coal 

 Measures of North America, Britain, etc. — have likewise been dis- 

 covered as far back in time as the Upper Silurian of Lanarkshire, 

 being represented by a small form which I named and described, 

 in 1868, as Neolimulus falcatus, having eight thoracic segments 

 apparently free and movable, but wanting the tail-spine, which 

 probably was developed later in life, or may have been represented 

 by an extremely short terminal plate, as we see is the case in 

 the young larval Limulus. 1 Thus the earliest fossil king-crab known 

 probably resembled closely the free-swimming larva of the living 

 king-crab as it leaves the egg. 



I shall have occasion to refer again to the characters of 

 Limulus when speaking of our present knowledge of the trilobites. 



As to whether the Eurypterida — with their evidently aquatic 

 branchiated respiration, their jaw-feet provided with swimming- 

 (not walking-) extremities — are in the direct line of ancestral rela- 

 tionship to the recent scorpions, I may refer again to my paper 2 ' On 

 some Points in the Structure of the Xiphosura,' etc. : — ' This is one 

 very strong argument, to my mind, in favour of the higher zoological 

 position of Pterygotus — that, being extremely larval in its anatomy, 

 it consequently possessed the capacity for further development, and 

 so has been modified and disappeared ' — its latest representatives 

 being met with in the Coal Measures, where the then earliest known 

 examples of fossil scorpions had also been found. But the dis- 

 covery, almost simultaneously, by Thorell and Lindstrbm in Gotland ; 

 by B. X. Peach in Scotland ; and by Whitfield in Xorth America (in 

 1885) of actual pulmonated land-scorpions in rocks of Upper Silurian 

 age (as far back, in fact, in geological time as the earliest-known 

 occurrences of Pterygoids, Slimonia, and Eurypterus) indicates that 



1 G-eol. Mag. vol. v. (1868) pi. i. p. 1. 



2 Quart. Journ. Geol. Soc. vol. xxiii. (1867) p. 35. 



