At Hallae is a two-headed mouse; the Conant Museum in Maine contains the skeleton of an adult sheep with two heads; there was an account of a two-headed pigeon published in France in 1734; Leidy found a two-headed snake in a field near Philadelphia; Geoffroy Saint-Hilaire and Conant both found similar creatures, and there is one in the Museum at Harvard; Wyman saw a living double-headed snake in the Jardin des Plantes in Paris in 1853, and many parallel instances are on record.
Classification .-- We shall attempt no scientific discussion of the causation or embryologic derivation of the monster, contenting ourselves with simple history and description, adding any associate facts of interest that may be suggested.

For further information, the reader is referred to the authors cited or to any of the standard treatises on teratology.
Many classifications of terata have been offered, and each possesses some advantage.

The modern reader is referred to the modification of the grouping of Geoffroy-Saint-Hilaire given by Hirst and Piersol, or those of Blanc and Guinard.

For convenience, we have adopted the following classification, which will include only those monsters that have LIVED AFTER BIRTH, and who have attracted general notice or attained some fame in their time, as attested by accounts in contemporary literature.
CLASS 1 .-- Union of several fetuses.

CLASS 2 .-- Union of two distinct fetuses by a connecting band.