The suspensory apparatus of the lens has been studied in two species of placental mammals (dog and horse) and in two species of marsupials (phalanger and wallaby). Particular attention has been paid to the suspensory apparatus of the dog and the phalanger. It has been found that the suspensory apparatus of the lens shows great morphological variability, although some features generally are more common in one species than in the others. For instance, in the dog there is a tendency to form fibrous bundles, an uneven distribution of fibres around the lens, and a fan-like arrangement of these fibres at their attachment to the lens. In the phalanger the fibres are evenly distributed around most of the circumference of the lens and the bundles, if formed, are slender. An area devoid of fibres, described as a diastema,is also present. The morphology of ciliary folds, and of the fibrous bundles and the various types of interconnections between them, have been described and illustrated, and the comparative morphology of the suspensory apparatus of the lens discussed. The reduction of the accommodative mechanism from that situated on both horizontal and vertical planes to that on a horizontal plane only has been discussed. The latter type is found in mammals. The mechanics of the suspensory apparatus of the lens has also been considered and attention given to the rotation of the ciliary folds and the twisting of the fibrous bundles, as observed in the dry specimens. A possible relationship of this finding to the adaptation of the lens has also been discussed. The suggestion has been put forward that the suspensory apparatus of the lens is not only suspensory but also regulatory in function, the regulation depending on its ability to disperse stresses between the ciliary muscle and the lens. The formation of bundles, the existence of interconnecting fibres and interconnecting nets, and the spreading of the fibres of a bundle at its attachment to the lens, are the main factors contributing to the regulation of stresses. The morphology of the suspensory apparatus of the dog and the phalanger was compared with that of man.