540 .MKRISTIC VARIATION. [PART i. 



< )n thr morphology or -i-niticance of duplicity in limbs I can 

 make no comim-nt li-y<'inl the few remarks given on p. 4UG. 

 It is just p<i-sil)le that in Xos. 832 to 834 the duplicity of tin- 

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

 Mi i mi' Symmetry; for some chelae are peripherally very nearly 

 -ymiiietrical about the plane of the dactylopodite and index. 



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

 ebrates, for tlimi^h in various works there are some scores of 

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

 In-ill^ almost certainly cases of double extra parts in Secondary 

 S\ mmetry having their duplicity disguised as we saw it in 

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

 tin -si- 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 

 then -f.>re 1 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- 

 tennae must in particular be borne in mind. 



CRUSTACEA. 



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

 Fig. 1!N, 11 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 

 \\cn- dorsal and ventral to each cither [cp. No. 503]. ALURKCHT, Sitzli. Ak. J I"/. -.>. 

 Ilerl., IHSI;, ]). .")!.",, 1>1. vi. Silurus glanis : extra tin attached to pelvic girdle and 

 partly to rt. pelvic fin. WAIU-ACHOWSKI, Anat. Auz., 1888, in. p. 379, J'KJ. Rana 

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

 M.'in. Ace. i:l!in'i, 1881, S. 4, in. p. 812, PI. iv. .//.'/ H. 



In Kaiiilie u group of cases of extra fin are known. They are upward projections 

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



790'' 

 tins, but in the- only case 1 have seen (/',///* .!/. X, [[., -1- - , kindly shewn me by 



A 



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

 -eeins, from external examination, to spring from some part of the pectoral girdle 

 (? left scapula). See LAC-KI^IU-: (who named such a tish "linja cucieri"), Hixt. nut. 

 dei Pom., 17'.i8, i. p. Ill, 7V. vii. ; Ni:n.i., M,-,n. WITH. 's,c., 1808, i. p. 554; 

 MoiiKAf, /'ii/*.<. <!>' I" France, issl, i. p. '200. In these fishes the real dorsal tin* 

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

 l>i-si-r. Aniiii. in itiu. (>i-i<-iit., 177-"i, i. JL 1H). This repetition is of course cpn'te 

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

 pectoral.^ are partis divided into two lobes (R. clavata, YARKELL, Brit. Finli., ed. 

 IJic-iiAKhsnv, is.v.i, u. )i. 586); or are separated from the head so as to project like 

 horns on cither side, as in lust case; and also in R. clavata, YAKKKI.L, ilnd.; p. 384; 

 DAY, /;;//. I-'i*li., u. ]<. '^'. I'l. CLTUU. fig. 'J : in R. batis, DAY, I.e., p. 337; in 

 R. asterias. Hi ui \i , Hull. sac. zool. Frum-i; 1889, xiv. p. 313, Jig. 



