88 



TJie Origin of Mammals. 



there may be a single condyle slightly divided or two exoccipital condyles. 

 There is, on passing from earlier to later types, a steady increase in the 

 size of the dentary and decrease in the size of the other elements of the 

 jaw. The quadrate also becomes much reduced in the higher types. In 

 Gorgonopsians and probably all earlier types the arch of the atlas is a 

 pair of bones ; in Cynodonts, as in mammals, there is a single arch. 



It is argued that the small Gorgonopsians fed almost exclusively on the 

 comparatively slow-moving, small, herbivorous Anomodonts. In the Trias 

 the small Anomodonts became very rare, and the carnivorous Therapsids 

 had to feed on other small forms, apparently the more active lizard- 

 like Cotylosaurs, such as Procolophon. The change of habit resulted in the 

 Cynodontia. 



In Upper Triassic times the larger Cynodonts preyed upon the large 

 Anomodont, Kannc.mcyeria, and carried on their existence so long as these 

 Anomodonts survived, but died out with them about the end of the Trias 

 or in Bhsetie times. The small Cynodonts, having neither small Anomodonts 

 nor small Cotylosaurs to feed on, were forced to hunt the very active long- 

 limbed Thecodonts. The greatly increased activity brought about that 

 series of changes which formed the mammals — the flexible skin with hair, 

 the four-chambered heart and warm blood, the loose jaw with teeth for 

 mastication, an increased development of tactile sensation and a great 

 increase of cerebrum. Not improbably the attacks of the newly-evolved 

 Cynodont or mammalian type brought about a corresponding evolution in 

 the Pseudosuchian Thecodonts which ultimately resulted in the formation 

 of Dinosaurs and Birds. 



A Case of Abnormal Trichromatic Colour Vision due to a Shift 

 in the Spectrum of the Green-Sensation Curve. 

 By Sir W. de W. Abney, K.C.B., F.R.S., and W. Watson, D.Sc, B.B.S. 



(Received June 13, — Bead June 26, 1913.) 



[This paper is published in 1 Proceedings,' Series A, vol. 89, No. 610.] 



