Considerable literature has accumulated dealing with observed cases of malformations resulting from arrest of development of the mandibular or the first branchial arch. Study has been given to these conditions not only as they exist in the human species but also as they occur in the other amniota. By far the most frequent conditions falling under this classification in man are micrognathia, cleft palate and harelip. These are examples of malformations existing in viable newborn infants. Among the teratologic examples of the nonviable are cyclopia and agnathia. While the latter are interesting, they do not engage the attention of the clinician. The condition of micrognathia is rather frequent among sheep and is called by shepherds "hog-jaw." Young lambs so afflicted cannot suckle or graze and are often destroyed for this reason. As early as 1822 Geoffroy-Saint-Hilaire1described the case of a sheep which showed micrognathia, conjunction of the ears, deformity