of egg-retention, egg-guarding and viviparity in single-brooded species. The discovery of varying degrees of egg-retention in a number of typhlopids bears on this problem. Both temperate (T. bibronii) and tropical (T. diardi) species display prolonged egg-retention, indicating that in typhlopids temperature may not be the main selective pressure in the evolution of viviparity. However, nothing is known concerning thermal requirements of these species. In addition other climatic factors, such as seasonal drought, may be involved. Acknowledgments.-We thank Wulf Haacke (Transvaal Museum) for permitting us to study specimens in his care.