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Show 424 DR. B. C. A. WINDLE 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 iEgypti fluvio, non procul ab oppido Latislana cognominato. Hie piscis magnitudinem fere Crocodili adsequabat, 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 tbe B. C. S. Museum. It must also have been enormously exaggerated, since there is no other description 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 bellies. In 1765, Jacobi (4) gave the first account of any importance of the double forms which he had observed in a fish-hatching establishment:-" E n faisant eclore des truites," he says, " j'ai quelquefois remarque quantite d'avortons ou de monstres, 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 serres l'un contre l'autre par le ventre." According to von Baer, Budolphi (5), Heusner (6), and Bathke (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 three 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 tbis class. I have not examined the median eye by sectional method, but from its external appearance it appears to be composed of two eyes fused together. 3. Four equal-sized eyes. I have one specimen of this condition myself (fig. 2), and it has also been described by Knoch (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 triplicity. 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 m y specimens fall into this class, instances of |