480 



MKRISTIC VARIATION. 



[PART i. 



to "Ventral" tin- two supernumeraries will turn their dorsal surfaces to each 

 i. :ui 1 .- on. The model SL thus rotates twice oil its own axis for each 



SK 



FlG. 1")H. A mechanical device for shewing the relations that extra legs in 

 Secondary Symmetry bear to each other and to the normal leg from which they 

 arise. The mode! /,' represents a normal right leg. Sf, and SR represent respect- 

 ively the extra right and extra left legs of the supernumerary pair. A and 1\ the 

 anterior arid posterior spurs of the tibia. In each leg the morphologically anterior 

 surl'aee i- -haded, the posterior being white. R is seen from the ventral aspect and 

 si. and SR are in Position VP. 



revolution round R, but the surfaces of the model SR always remain parallel to' those 

 of the model R. In every possible position therefore each model is the image of its 

 hbour in a mirror tangential to the circle of revolution. In the figure the models 

 ha\e tin jMi-iiion they should have if arising postero-ventrally. Here the plantar 

 surface of SL is at right angles to the plantar surfaces of the other two legs. 



Since at each radius the relative position of the legs differs, it is 

 jms.sible to define these positions l>y naming the radius. This will be 

 dene as shewn in Fig. 154. In this diagram imaginary sections of the 

 legs are shewn in the various positions they would assume at various 

 radii. The central thick outline shews a section of the normal leg, a 

 lunger process distinguishing the anterior surface from the posterior. 

 'I'll.' radii an- drawn to various points D, A, V, P, representing 

 the dorsal, anterior, ventral and posterior positions respectively. In- 

 termediate positions may be marked by combinations, DA, VVP, 

 &C., using (he system employed in boxing the Compass. 



<>n several of the radii ideal sections of the extra lei's are shewn in 



O 



i liin lines, the shaded one being the nearer and the plain one the 

 remoter. .!/' ;uul M~ >he\v the planes of the imaginary mirrors. 



The manner in which the pair of extra limbs are compounded with 

 each other in their proximal parts, and with the normal limb at their 



