DOUBLE MONSTROSITY— IN CARTILAGINOUS FISHES 31 



margin. Taking the figures as they stand, they would mean that in order to remain separate the 

 two embryonic rudiments would have to be located, to begin with, at an angular distance of not less 

 than 129'5 from one another on the margin of the blastoderm. The corresponding distance in the 

 trout, as estimated in the same manner, amounts to not less than 165° (p. 7). 



Classification of Double Monster Cartilaginous Fishes. 



Probably if a full series were available for comparison, the same types of double monstrosity 

 would be found to occur in the cartilaginous as in the osseous fishes. The mode of origin will be 

 essentially the same in the one group as in the other, and the degree of duplicity will again depend 

 on the distance which originally separated the two embryonic rudiments on the margin of the 

 blastoderm. On these points, however, we must speak by inference, since direct observations are 

 a wanting. 



Altogether I have only been able to collate thirteen recorded instances in the cartilaginous 

 fishes. The chances are that this number should be reduced to twelve or possibly to eleven, owing 

 to duplications, no longer traceable, in the records. As a rule few descriptive details are given, even 

 regarding external characters, and in no case is the internal anatomy adequately outlined. None of 

 the specimens is described as katadidymous, although that figured by Gadeau de Kerville (see below) 

 somewhat resembles the later stages of this condition in the trout. No instance of triplicity seems 

 yet to have been observed except under experimental conditions (see note on p. 5). The records 

 which 1 have been able to obtain are given below. In regard to some of the specimens, the species 

 and even the genus cannot now be ascertained, but all such specimens may be put down as being 

 sharks or dogfish. 



Union in the Read Region. An example is figured by St. Hilaire (313, PL XIV. 5) in the 

 atlas attached to his great work. The two inner eyes are close together, but do not seem to be 

 actually united. As far as one can judge, inner spiracle-openings are absent. Probably the twin 

 brains are united with one another towards the anterior part of the medulla. 



Union in the same region or a little further back is perhaps exemplified by the instance recorded 

 by Risso (205), in which two mouth-openings were present, one above the other. St. Hilaire, 

 however (III. 202), throws some doubt on the validity of this record. 



Union in Pectoral Region. Gadeau de Kerville (73, PI. XVIII. figs. 1,2). A young Acanthias 

 vulgaris with the umbilical vesicle still well marked externally. Inner pectoral fins are not present. 

 The posterior part of the body is irregular and distorted in a manner resembling, superficially at 

 least, the condition frequently seen in advanced katadidymous trout embryos. 



Union just behind the Pectoral Region. Quatrefages (198 pp. 11-12) gives a careful description 

 of a young Acanthias showing anterior duplicity. The union occurs just behind the pectoral region. 

 Inner pectoral fins are present, the one belonging to the right twin being markedly displaced down- 

 wards, while the other is correspondingly displaced in an upward direction. 



Union near the middle of the Body. Aldrovandi (;?). The shark referred to on p. 2. 



Heusner (94 8 p. 34). " Dr. Barclay of Edinburgh had a new-born specimen, double to the 

 umbilicus, while there is a similar specimen in the Hunterian Museum " (quoted from v. Baer 5 p. 88). 



Lowne (14-6 y>. 11 No. 23). A foetal dog-fish exhibiting anterior dichotomy. The dichotomy 

 extends to the umbilical region, and the ventral convergence of the sagittal planes is so well marked 

 that the component embryos may almost be described as facing one another. 



Union by the yolk-sac only, or, after absorption of the yolk, by the abdominal walls. 



Heusner (94- 8 p. 34). " Professor Brookes in London had a specimen double anteriorly and 

 posteriorly, but united in the middle of the body " (quoted from v. Baer 5 p. 88). 



Levison. A sternopagous double monster (quoted from Panum 180 p. 72). 



Lowne (14-6 p. 11 No. 22) catalogues two small sharks united by the abdominal walls. The 

 remains of a single yolk-sac is seen between the pectoral fins. The specimen was obtained from a 

 female shark taken by the donor (mate of a vessel) in the Indian Ocean. When brought on deck 



