THE TRILOBITA AND OTHER ARTHROPODA 219 



trilobites, but this is largely the effect of adaptation 

 to similar conditions of life. 



The Scorpiones are direct descendants of the 

 eurypterids, from which they differ only in the effects 

 of adaptation to a land-life. They are known from the 

 Silurian period, and the scorpions of to-day differ very 

 little from those of that early time. 



The Araneae (spiders) are derived from scorpions by 

 a shortening of the abdominal region. They are known 

 from the Coal Measures, but are very rare as fossils until 

 the Oligocene, where they occur in the amber (fossil 

 resin) of the Baltic area. 



The Myriopoda (millipedes and centipedes) are very 

 rare as fossils, but occur as early as the Devonian. 



The Insecta are abundant in occasional beds, of which 

 the Coal Measures (particularly of Commentry in France) 

 and the Oligocene amber are the most prolific, but other- 

 wise very rare. The earliest is recorded from the Upper 

 Ordovician, and if not misidentified is the only land- 

 animal known from so early a period. 



Short Bibliography. 



BEECHER, C. E. (i) Outline of a natural classification 

 of Trilobites, Amer. Journ. Sci. (4), vol, iii. (1897). 



(2) A series of articles on appendages of Triarthrus and 

 Trinucleus, Amer. Journ. Sci. and Amer. Geologist, 1893-96. 



(3) Larval stages of Trilobites, Amer. Geologist, vol. xvi. 



(1895). 



DOLLO, L. La Paleontologie Ethologique, Bull. Soc. 



Beige Geol. Pal. Hydr., vol. xxiii. (1909). 



RAYMOND, P. E. Beecher's Classification, Amer. Journ. 

 Sci., vol. xliii., p, 208 (1917). 



REED, F. R. C. Numerous papers in Quart. Journ. 

 Geol. Soc. (from 1896) and Geol. Mag. (from 1894). 



