724 



HISTORICAL GEOLOGY. 



the Myriapod type, and beyond this, to the memberless condition of the 

 Worm. This accords with a common fact that the higher the species, the 

 longer the stage of yonth. 



The relations in body segments and limbs between the classes of Crusta- 

 ceans, Limuloids, Arachnids, Myriapods, and Insects, are shown in the 

 following table. The segments of the parts of the body are numbered along 

 the left margin ; the zero opposite signifies that the segment, though present, 

 has no appendage. 



CRUSTA- 













MYRIA- 







CEANS 





LIMULOIDS 



ARACHNIDS 





PODS 





















INSECTS 





















Tetradecapods 



Eurypterus 



Pterygotus 



Limulus 



Scorpion 



Plirynus 



Lithobius 







1. 1st Ant. 1 





1 





Ant. ] 



Ant. 





Falces 



73 



Falces ' 



13 

 03 



Ant. 1 



'o 



Ant. 



13 



2. 2d Ant. 





M-P. 





M-P. 





M-P. 





M. 





M. 



iTl 



M. 





M. 





3. M. 



S 



M-P. 





M-P. 



ID 



M-P. 



73 



P. . 



P. . 



X 



Mx. J 



W 



Mx.&L. J 



W 



4. Mx. 



K 



M-P. 



M 



M-P. 



K 



M-P. 



ffi 



P. 





P. 



a 



p -1 





p ^ 



X 



5. Mx. 





M-P. 





M-P. 





M-P. 





P. 





P. 



o 



P. 





P. 





6. Mx. J 





M-P. J 





M-P. ; 





M-P. J 





P. J 





P- J 



rH 



P. 

 P. 





P. 



H 



1. P. 1 





Fol. P. 1 





Fol. P. ^ 





Fol. P. ] 





) 



o 

 H 









1 





2. P. 





Fol. P. 





Fol. P. 





Fol. P. 





Comb 









P. 











3. P. 

















Fol. P. 















Oi 



S 

 o 



T3 

 < 



P. 



d 



s 



o 

 < 









4. P. 



5. P. 



6. P. 



7. P. J 



o 





 

 

 , 



o 



H 





 

 

 . 



o 



Fol. P. 

 Fol. P. 

 Fol. P. 



J 



O 





 

 

 







 

 

 



P. 

 P. 

 P. 

 P. 





 

 

 



13 



a 



o 



1. App. ^ 





1 





1 





1 





1 







 ^ 





P. 

 P. 

 p 







 





2. App. 









£ 







a 





5 







s 











3. App. 



s 







r^ 







a 





s 







fl 















4. App. 



5. App. 



o 



< 





 



o 

 < 





 



o 



-a 



< 





o 



< 





 



o 

 < 







P. 

 P. . 









6. App. 





J 





J 









J 































(?5 O 13 













































:3 g.3 























pairs 

 thobi 

 pend 

 Myr 























15 

 in Li 



Scolo 

 some 







In this table, the following abbreviations are used: Ant., antenna; App., pairs of 

 jointed appendages, either pediform or branchial ; M., mandible ; Mx., maxilla ; P., feet ; 

 M-P., feet that serve also as jaws ; Mx. & L. (under Insects), maxillse and labium ; Fol. 

 P., foliaceous or lamellar feet or appendages. 



Under the Limuloids, the genus Eurypterus fails of antennae ; but they 

 are present in Pterygotus, and are chelate ; and this chelate (or thumb-and- 

 finger) form characterizes also the modern Limulus, the Scorpions, and the 

 common Spiders. In the table, the two pairs of maxillse of Insects are 

 assumed to belong to a single body segment, as held by many zoologists, 

 including (as he himself informs the author) S. I. Smith; the table shows 



