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D'ARCY WENTWORTH THOMPSON ON 



the carapace of these crabs presents a somewhat triangular form, which seems at 

 first sight more or less similar to those just described, we soon see that the actual 

 posterior border is now narrow instead of broad, the broadest part of the carapace 

 corresponding precisely, not to that which is broadest in Paralomis, but to that 

 which was broadest in Geryon ; while the most striking difference from the latter 

 lies in an antero-posterior lengthening of the forepart of the carapace, culminating 

 in a great elongation of the frontal region, with its two spines or " horns." The 

 curved ordinates here converge posteriorly and diverge widely in front (figs. 22 

 and 23), while the decremental interspacing of the abscissae is very marked indeed. 



Fig. 24. — Harpinia pluwosa, Kr. 



'ig. 25. — K/egoce/iJiahis ivflatus, Kr. 



Pig. 26. — Hyperia galba. 



We put our method to a severer test when we attempt to sketch an entire and 

 complicated animal than when we simply compare corresponding parts such as the 

 carapaces of various Malacostraca, or related bones as in the case of the tapir's toes. 

 Nevertheless, up to a certain point, the method stands the test very well. In 

 other words, one particular mode and direction of variation is often (or even usually ) 

 so prominent and so paramount throughout the entire organism, that one comprc 

 hensive system of co-ordinates suffices to give a fair picture of the actual phenomenon. 

 To take another illustration from the Crustacea, 1 have drawn roughly in fig. 24 a 

 little Amphipod of the family lMioxocephalidse (Harpinia sp.). Deforming the oo 

 ordinates of the figure into the curved orthogonal system in fig. 25, we at once 

 obtain a vcrv fair representation of an allied genus belonging to a different family 

 of Amphipods, namely, Stegocephalus. As we proceed further from our type our 



