There has been no general agreement regarding the presence of arachnoid granidations and arachnoid villi in mammals. This seems to have arisen from the fact that various workers have used different criteria in distinguishing between the two terms — 'granula tion' and 'villus'. Luschka (1852) was the first to. define the structure observed macroscopically as an 'arachnoid granulation', while for the microscopic structure he used the term 'arachnoid villus', a definition also used more recently by Turner (1958). It seems, therefore, that the only difference between these two structures is one of size, for it was Hassin (1930) who defined an arachnoid granulation as a hypertrophied villus consisting of several lobules, covered by a membrane and possessing a common stalk. Le Gros Clark (1920) regarded the structures seen with a hand lens also as granulations whereas Cooper (1958) used the term granulation to include all these structures irrespective of size.