n. plLissencephalalĭs`sĕn*sĕf"ȧ*lȧ(Zoöl) A general name for all those placental mammals that have a brain with few or no cerebral convolutions, as Rodentia, Insectivora, etc.
Century Dictionary and Cyclopedia
lissencephalaThose mammals which have smooth brains; in Owen's system of classification, one of four prime divisions of Mammalia. The corpus callosum is present and well developed (as it is not in Lyencephala), but the cerebral hemispheres are small, leaving much of the olfactory lobes and of the cerebellum uncovered, and their surfaces are smooth, having slight, few, or no convolutions (as is not the case in Gyrencephala and Archencephala). The Lissencephala comprise the Bruta or edentates, Chiroptera or bats, Insectivora, and Rodentia. The group thus corresponds to the Ineducabilia of Bonaparte and Microsthena of Dana, or the lower series of placental or monodelphous mammals, as Gyrencephala does to the higher series Educabilia. Owen's Lyencephala were the marsupials and monotremes, or didelphian and ornithodelphian mammals; his Archencephala included man alone. The lissencephalous brain is illustrated under gyrus (fig. 1).
Webster's Revised Unabridged Dictionary
NL., fr. Gr. lisso`s smooth + 'egke`falos the brain