The therapsids are usually placed in the class Reptilia as its most mammal-like representatives. This is on the basis of the now commonly accepted criteria of jaw-ear structure, where mammals are defined as having (1) a dentary-squamosal suspensorium, (2) the articularquadrate joint not a mandibular suspensorium, (3) three middle ear bones, and (4) the mandible containing only the dentary (Simpson 1959). It is the present thesis that (1) tetrapod classes should be defined on the basis of their major adaptive differences, (2) that the mammalian grade of adaptation was largely reached in the therapsids, and (3) that the therapsids should therefore be included in the Mammalia. No specific mammal or reptile need be referred to; comparison will rather be with a construct similar to the Typus of the German idealistic morphologists, whose characters are observable in most species except for obvious specializations. The Theria as a whole and the Sauropsida (of Watson 1957) as a whole are the standards of comparison. If necessary, Diadectes, Iguana, Didelphis, and Sits may be kept in mind. Much of the evidence for this view has been discussed by Brink (1957a); see also Broom (1932). The major adaptive distinctions of mammals as compared to typical reptiles will now be considered, together with any possible evidence that the therapsids had each of these characters in life. "The theoretical approach to any field of science is something of a luxury" (Burnet 1959). However, if the criteria of class placement are to be