136 REPORT OF COMMISSIONERS OF IKLAND FISHERIES. 



a. Extra process repeating but one segment, i. e., the distal seg- 

 ment or dactyl. 



b. Extra processes repeating more than one segment. 



2. Extra Processes Repeating hut One Segment, i e., the Distal Segment 

 or Dactyl. 



Specimens Nos. 1 and 2. 



Bateson's rules for secondary symmetry will apply to extra pro- 

 cesses arising from any morphological surface of the normal limb; 

 i. e., arising from the morphologically ventral, dorsal, anterior, and 

 posterior surfaces, or from an intermediate position. Accordingly, 

 given the morphological surface origin of the extra processes, we can 

 state theoretically what the spatial relations will be. Now for speci- 

 mens Nos. 1 and 2, the abnormal structures in the former arises from 

 the smooth or morphologically dorsal surface of the dactyl, and in 

 the latter the origin is from the toothed or ventral border of the same 

 segment. The following diagram has been drawn to show what the 

 relations for these two positions should theoretically be for the right 

 dactyl of a lobster claw. 



In the figure at the left of this diagram. Fig. I represents a cross 

 section of a dactyl taken through the dorso-ventral plane. Figs. 

 II and III each represent cross sections for a pair of extra processes 

 arising, respectively, from the dorsal and ventral surfaces of a normal 

 dactyl; the section in each case is in the dorso-ventral plane and 

 through points .homologous to those of the normal section in Fig. I. 

 In each of these sections the pointed portion represents the toothed 

 or ventral border, and opposite oval portion represent the dorsal or 

 smooth border of the dactyl; the light and shaded areas, respectively, 

 indicate the anterior and posterior sides of the segment. It will be 

 observed that these sections have been placed in the relations de- 

 manded by Bateson's rules; for in each pair of extra parts (Figs. 

 II and III) "the nearer of the two extra appendages" (D'L') is the 

 mirror image of the normal appendage (DR), and " the remoter 



