PREFACE. 



Since, of late years, the conclusion has become very generally accepted among 

 Naturalists that the development of the embryo of any animal is but a recapitula- 

 tion of the ancestral history of the species to which it belongs, a new impulse has 

 been imparted alike both to embryological and palasontological research, for each 

 furnishes most valuable and independent evidence by which to demonstrate the 

 correctness or fallibility of the doctrine of Evolution. 



But whilst, on the one hand, the reconstruction of the pedigree of a group from 

 the developmental history of its existing members is admittedly fraught with 

 difficulties — for the series of developmental stages of the individual organism 

 probably never presents more than an abbreviated and condensed summary of 

 ancestral conditions, whilst the summary itself is oftentimes strangely modified — 

 on the other hand, the palaeontologist is only too well aware that the materials at 

 his command merely enable him to furnish an outline of the ancestry of many of 

 the groups most largely represented at the present day. 



There is, perhaps, no group among the fossil Crustacea which affords such 

 ample materials for comparison with recent forms as the order Meeostomata. 



Extending back in time to the Lower Silurian Rocks, these ancient forms not 

 only portray the larval stages of the highest forms of living Crustacea, but they 

 also foreshadow the incoming of the air-breathing Aeachnida, whose ancestors 

 they probably were. 



Another interesting feature of the order is its bipartite division into 

 brachyuran and macrouran forms which exemplify the crawling and swimming 

 types, by the soldering together of the body-segments in the one (Limulus), and 

 the retention of free movement in the somites of the other (Pterygotus). 



Numerous species of the ancient extinct long-bodied Eueypteeida have been 

 met with and described by Prof. James Hall in America, by Fischer and Niesz- 

 kowski in Russia and Sweden, and by Huxley and Salter and by myself in 

 Britain. 



The most perfect specimens of the genera Slimonia, Pterygotus, and Eurypterus, 

 have been obtained by Dr. J. Slimon, of Lesmahagow, Lanark. The largest 

 known remains, representing specimens from five to six feet in length, are from 

 the Old Red Sandstone of Forfarshire, belonging to the great Pterygotus anglicus 



