Scientific Teratology.--About the first half of the eighteenth century what might be called the positive period of teratology begins. Following the advent of this era come Mery, Duverney, Winslow, Lemery, and Littre. In their works true and concise descriptions are given and violent attacks are made against the ancient beliefs and prejudices. From the beginning of the second half of the last century to the present time may be termed the scientific epoch of teratology. We can almost with a certainty start this era with the names of Haller, Morgagni, Geoffroy-Saint-Hilaire, and Meckel, who adduced the explanations asked for by Harvey and Wolff. From the appearance of the treatise by Geoffroy-Saint-Hilaire, teratology has made enormous strides, and is to-day well on the road to becoming a science. Hand in hand with embryology it has been the subject of much investigation in this century, and to enumerate the workers of the present day who have helped to bring about scientific progress would be a task of many pages. Even in the artificial production of monsters much has been done, and a glance at the work of Dareste well repays the trouble. Essays on teratogenesis, with reference to batrachians, have been offered by Lombardini; and by Lereboullet and Knoch with reference to fishes. Foll and Warynski have reported their success in obtaining visceral inversion, and even this branch of the subject promises to become scientific. Terata are seen in the lower animals and always excite interest. Pare gives the history of a sheep with three heads, born in 1577; the central head was larger than the other two, as shown in the accompanying illustration. Many of the Museums of Natural History contain evidences of animal terata. 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.