CHAP, xxii.] DOUBLE APPENDAGES : ARTHROPOD A. 



543 



FIG. 202. Xantho punctulatus. Two views of left chela of No. 838, shewing the 

 division of the index. (After Herklots.) 



podite is wrinkled and has two apical articulations, each bearing a small 

 chela ; both are soft and not calcined, having articulations indicated 

 by furrows only. [No information as to planes.] RICHARD, Ann. Sci. 

 Nat., 1893, p. 106. 



841. Homarus americanus : right chela having a short articulated 

 process below the dactylopodite moving in plane at right angles to it. 

 [la double structure]. FAXON, Harv. Bull., vin. PI. i. tig. 12. 



842. H. americanus : toothless process articulating belcnv dactylopo- 

 dite, moving in plane at right angles to its plane of motion. It articu- 

 lates upon a separate process given by the propodite. [It is difficult 

 to suppose that this extra process can be double.] FAXON, /. c., PI. I. 



tig. 



16. 



Mr G. DIMMOCK of Canobie Lake, N. H. has kindly sent me word of a Gelasimus 

 having a chela of very anomalous form. Both index and dactylopodite are said to 

 have been bifid, but the plane of division was at right angle* to the plane of the 

 dactylopodite and index, so that all four points were in one plane. This specimen 

 has unfortunately been destroyed ; but Mr Dimmock tells me that the arrangement 

 was certainly thus, and that the unusual difficulty of bringing this case into agree- 

 ment with others was recognized in examining it. 



INSECTS. 



Among the following 110 cases which all either have been 

 or might be called cases of " duplicity " of legs, antennae, or 

 palpi, there is, I think, not one clear case of unmistakeable 

 duplicity, such as for instance those of the chelae in Nos. 831 

 or 832. They should thus be considered as cases in which the 

 extra parts have not been or cannot be shewn to be double, 

 rather than as examples of proved duplicity of normal appendages. 

 In every case that I have myself properly examined, it is either 

 possible to prove the duplicity of the extra parts ; or else essential 

 features (e.g. spurs &c.) by which a right appendage may be 

 told from a left are wanting. Nevertheless the few straight- 

 forward cases of double-limbs in Crustacea keep one alive to the 

 possibility that some of these also may be the same. The most 

 probable cases of true duplicity of limbs are Nos. 844, 846 and 851. 



