424 DB. B. c, A. wijfDLE ON DOUBLE [May 21, 



making mention of a few of the earlier notices on the point, to 

 which I shall not again have occasion to revert. As far as I know 

 the earliest description of a double fish is that given by Aldro- 

 vandus (1) : — " captus fuit," he says, "in Nilo Jigypti fluvio, non 

 procul ab oppido Latislana cognominato. Hie piscis magnitndinem 

 fere Crocodili adaequabat, coloris erat leucopha?i albicantibus maculis 

 insigniti. Habebat quidem duo capita etc." This description, 

 which seems to be original to Aldrovandus, since I have been unable 

 to find it in any of the other similar works which I have searched, 

 must have been based upon the description of some double fish, 

 perhaps a Shark, like that in the E. C. S. Museum. It must also 

 have been enormously exaggerated, since there is no other de- 

 scription of a fish having lived to attain any but a small size. 

 According to von Baer (2), Jussieu, in 1754(3), exhibited to the 

 French Academy two small fishes united by their belhes. In 

 1765, Jacobi (4) gave the first account of any importance of the 

 double forms wliich he had observed in a fish-hatching establish- 

 ment: — " En faisant eclore des truites," he says, " j'ai quelquefois 

 remarque quantite d'avortons ou de inonstres, certaines annees plus, 

 d'autres moins. Quelques-uns avaient deux tetes et le corps bien 

 forme ; d'autres avaient le ventre commun et du reste etaient deux 

 poissons bien distincts comme seraient deux poissons ordinaires 

 que Ton coucherait sur une table bien serre's I'un contre I'autre par 

 le ventre." According to von Baer, Eudolphi (5), Heusner (6), and 

 Eathke (7) have mentioned similar forms, but I have been unable 

 to refer to the original papers. 



II. Observed forms of Duplicity. 



1. Three eyes of same size (Plate XXV. fig. 1). I have thi-ee 

 specimens in which the head is somewhat broader than usual, and 

 is provided with a third, median eye, which appears from external 

 examination to be of the same size as those to each side of it. I 

 have not found this condition, which seems to be the least 

 manifestation of duplicity, mentioned in any of the papers to 

 which I have referred. 



2. Three eyes, the median being larger than either of the lateral. 

 I have one specimen of this class. I have not examined the median 

 eye b}' sectional method, but from its external appearance it appears 

 to be composed of two eyes fused together. 



3. Foul" equal-sized eyes. I have one specimen of this condition 

 myself (fig. 2), and it has also been described by Enoch (8) and by 

 Klaussner (9). In the instance given by the last-mentioned writer, 

 the fish had two distinct heads, one of which was provided with 

 two, the other with four eyes, the condition thus being one of 

 triphcity. Knoch states that there may be two mouths in cases 

 of the kind included in this class. Such is not the case in my 

 specimen. 



4. Two heads, the duplicity extending as far back as the otic 

 region. None of my specimens fall into this class, instances of 



