174 M ERISTIC VARIATION. [PART i. 



HI. B. polytrema : single specimen from Chili, badly preserved 

 hut apparently having fourteen pairs of gill-openings. GUNTHER 

 I.C., p. .'.12. 



Sp. cimen having 14 ;_a 11 -openings on left side and 13 on right. 

 SCHNEIDER, A., Arc-It, f. Nuturg., XLVI. 1880, p. 115 (cp. PUTNAM, 

 /'/ li '. N. II. &, xvi. 1873, p. 100). 



/;. bischoffii: single specimen, 10 gill-openings on each side. 

 ibid. 



Ammocoetes : having eight branchial openings on each side instead 

 of seven, the normal number. The shape of the mouth of this specimen 

 also .diiiormal, heing described as somewhat square. [No satis- 

 t'.u-tory description.] EDWARD, THOMAS. Zuoloyist, xvi., p. 6097. 



1 [-2. In connexioD with individual Variation in the number of gill- 

 saca in Myxinoids it should be borne in mind that in Petro- 

 myzon tin -re are normally seven pairs of gill-sacs. The case of the 

 Notidanidie may also be mentioned in this connexion. Among 

 Selachians tin- Nut idanidse are peculiar in having a number of gill- 

 slits iitluT than live, and of them Hexanchus has six pairs, while 



///,/, ///(////.- ha.- seven 1 . 



III. CERVICAL FISTUL.E AND SUPERNUMERARY AURICLES IN 



MAMMALS. 



Though tin- evidence of this subject is well known and has 

 often been collected, it may be convenient to give here some 

 .detract of the farts in so far as the phenomena of Variation are 

 illustrated by tln-m. Since cervical fistula; have been believed to 

 result frci 1 1 tin- | H ! sistence of the embryonic branchial clefts, they 

 may properly IK- considered in relation to the general question of 

 Variation in the numher of gill-slits, while the development of 

 external appendages pei'hap- Anally homologous with the external 

 ears, directly concerns the subject of Meristic Variation. 

 Man. The .subject has been studied by many observers, espe- 

 cially by ASI-III:I;SII\-, and by HEUsiNGER 3 , who brought together 

 and abstracted 4<i cases, being all that had been described in Man 

 up to isiii-. <;. Kis< IIKR* gives a full list of the literature of the 

 subject up to |N7(>, with an analysis of Go cases. A further paper 

 liy ! 1 1:1 siNci.i; contains a general account of these structures as 

 'he\ are toiind in Man and in the domestic animals. Additional 

 casi 8j i.> ;j , ther with a general discussion of the subject, especially 

 in relation to tistuL-eon t he external ears, were given by Sir JAMES 



' Balanoglossus. In li\v sjiccios with which I am acquainted, the number 

 'f ^'ill-liars ami >lits varies in prnportion to the size of the l><uly, and as it is not 

 unlik'-ly that tln-c animals coniinn.' to LMI>\V tlirnu^lmut life, it is probable that the 

 miiiilirr i if ln-aii'-hia is always increasing by formation <>t new ^ill-slits at the 

 posterior end d" ill.' branchial re^imi. The same is probably true of Amphioxiis. 

 \ - .11 toast, I>,' ii<tii!i.< f.,/// ,,,!!./, H/li'. Hnlin. lsH'2. 

 H Irch.f. I'.it/i. .ln>it. a. I'ln/.t.. 1st; i, \\ix. 



I i-' in n, (.. Dent, /.t.<.<-li.f. clunir;!., I.s7:i. 

 Hi V-IN..I.I:. li.nt. Zttch. f, Th ic nn., 1*78. 



