XVII] THE COMPARISON OF RELATED FORMS 745 



direction, such as Matuta or Lupa (5), and conversely, by 

 transverse compression, to such a form as Corystes (2). In 

 certain other cases the carapace conforms to a triangular dia- 

 gram, more or less curvilinear, as in Fig. 4, which represents 

 the genus Paralomis. Here we can easily see that the posterior 

 border is transversely elongated as compared with that of Geryon, 

 while at the same time the anterior part is longitudinally extended 

 as compared with the posterior. A system of slightly curved and 

 converging ordinates, with orthogonal and logarithmically inter- 

 spaced abscissal lines, as shown in the figure, appears to satisfy 

 the conditions. 



In an interesting series of cases, such as the genus Chorinus, 

 or Scyramathia, and in the spider-crabs generally, we appear to 

 have just the converse of this. While 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. 3 and 6), while 

 the decremental interspacing of the abscissae is very marked 

 indeed. 



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 comprehensive system of co-ordinates 

 suffices to give a fair picture of the actual phenomenon. To take 

 another illustration from the Crustacea, I have drawn roughly in 

 Fig. 370, 1 a little amphipod of the family Phoxocephalidae 

 {Harpinia sp.). Deforming the co-ordinates of the figure into the 



