540 MERISTIC VARIATION. [part i. 



On the morphology or significance of duplicity in limbs I can 

 make no comment beyond the few remarks given on p. 406. 

 It is just possible that in Nos. 832 to 834 the duplicity of the 

 chela or of the index is a division in the middle line of a Bilateral 

 Minor Symmetry ; for some chelae are peripherally very nearly 

 symmetrical about the plane of the dactylopodite and index. 



In Arthropods double-limbs are no less rare than in Vert- 

 ebrates, for though in various works there are some scores of 

 cases to be found, the great majority may be safely rejected as 

 being almost certainly cases of double extra parts in Secondary 

 Symmetry having their duplicity disguised as we saw it in 

 Nos. 750, 764, or 801. By most of those who have dealt with 

 these things the possibility of disguised duplicity in the extra 

 part has been unheeded ; and ignorant of the special difficulties 

 of these cases they have thus set down specimens as examples 

 of duplicity of appendages at a casual glance. For this reason 

 therefore I shall only give particulars of those few cases which 

 are better established or otherwise of special interest, letting the 

 rest follow as a list of references. 



It will not be forgotten that whenever an extra part is in 

 itself symmetrical it always may be a double structure, and the 

 special application of this fact to cases of extra filamentous an- 

 tennas must in particular be borne in mind. 



Crustacea. 



*831. Hyas araneus : a left chela having the form shewn in 

 Fig. 198, II and III. Fig. 198, I shews a normal left chela of 

 this species from the outside in the same position as II. In 

 the abnormal specimen the dactylopodite D is normal save that 



pectoral fin double, the division being in a horizontal plane, so that the two filaments 



were dorsal and ventral to each other [cp. No. 503]. Albrecht, Sitzb. Ak. Wiss. 



Bed., 1886, p. 545, PL vi. Silurus glanis : extra fin attached to pelvic girdle and 



partly to rt. pelvic fin. Warpachowski, Anat. Anz., 1888, in. p. 379, fig. Rana 



esculenta : left hind foot double ; rt. not seen [a very clear case]. Ercolani, 



Mem. Ace. Bologna, 1881, S. 4, in. p. 812, PI. iv.fig. 11. ' 



In Kaiidae a group of cases of extra fin are known. They are upward projections 



from the dorsal surface near the middle line. They are often spoken of as " dorsal ' : 



7902 

 fins, but in the only case I have seen (Paris Mus. N. H., —^ , kindly shewn me by 



Prof. L. Vaillant) the attachment is not really median but is slightly oblique, and 

 seems, from external examination, to spriug from some part of the pectoral girdle 

 ('? left scapula). See Lacepede (who named such a fish "Raja cuvieri"), Hist. not. 

 des Poiss., 1798, i. p. 141, PL vn.; Neill, Mem. Wern. Soc., 1808, i. p. 554; 

 Moreau, Poiss. de la France, 1881, i. p. 206. In these fishes the real dorsal fins 

 were in the proper place (though in some species they may be far forward, Forskal, 

 Descr. Anim. in itin. Orient., 1775, i. p. 18). This repetition is of course quite 

 distinct from that other curious and also Discontinuous variation in which the 

 pectorals are partly divided into two lobes (R. clavata, Yarrell, Brit. Fish., eel. 

 Richardson, 1859, ii. p. 585) ; or are separated from the head so as to project like 

 horns on either side, as in last case ; and also in R. clavata, Yarrell, ibid. ; p. 384 ; 

 Day, Brit. Fish., n. p. 345, PL clxxi. fig. 2; in R. batis, Day, l. c., p. 337; in 

 R. asterias, Bureau, Bull. soc. zool. France, 1889, xiv. p. 313, fig. 



