536 



MERISTIC VARIATION. 



[part I. 



<?n'+L') 



Fig. 193. I. Right chela of C. pagurus in Coll. Surg. Mus. R, right index. 

 II. Similar specimen whose dactylopodite bears x, a supernumerary process. In 

 Coll. Surg. Mus. III. Astacus fluviatilis, left chela bearing a?, a supernumerary 

 process. EI, ED, right index and dactylopodite. (After Lucas.) 



both its faces in this plane. There is however no doubt that the 

 distinction between these cases and true duplicity is hard to trace 

 and possibly enough it is not really absolute. 



825. As each case differs from the others I give a list of those not in private col- 

 lections 1 . The ? indicates that the case perhaps approaches the condition of 

 true duplicity. 



D, dactylopodite. I, index. 



Tiedemann, Meckel's Arch., 1819, v. p. 127, 



PL v.. fig. 2. 

 JaejEE, (j., Jahresh. Ver. vaterl. Naturk., 



1851, xvii. p. 35, PI. i. fig. 7. 

 id., Meckel's Arch., 1826, p. 95, PI. u.fig. 3. 

 Rosel v. Eosenhof, Ins.-Belust., in. p. 344, 



fig. 31. 

 ibid., fig. 30. 

 Lucas, Ann. soc. ent. Fr., 1844, Ser. 2, n. 



p. 45, PI. i. fig. 6. 

 Faxon, Harv. Bull., vin. p. 259, PL i.Jig. 11. 

 ibid., PL i. fig. 6. 



Richard, Ann. sci. nat., 1893, p. 106. 

 Coll. Surg. Mus. 

 Coll. Surg. Mus. 



(e) Exceptional Gases. 



'82G. Homarus americanus : Right chela. Meropodite sub- 

 cylindrical instead of flattened ; peripherally divides into two 

 parts each bearing an articulated appendage as shewn in Fig. 194-. 

 [The appendage R is a normal chela. What is R' -t- L' ? Faxon, 

 carefully describing the case, thinks that R' + U is a rudimentary 

 and reversed copy of R, and that the case is one of duplicity. 

 But from the particulars given, and especially from the circum- 

 stance that the carpopodite was " much more spiny " than the 

 normal, I think it likely that R' + L' is morphologically a double 

 structure formed of a pair of carpopodites compounded together. 



1 With these may perhaps be mentioned the following : Apus cancriformis, 

 having upon the 40th foot a second small nabcllum shaped like the normal flabellum. 

 The bract was greatly reduced in size. Lankestkh, E. R., Q.J. M.S., 1881, xxi. p. 

 350, PL xx. fig. 12. [In explanation of Plate the abnormal foot is called the 30th.] 



