44© EVOLUTIONARY MORPHOLOGY 



appear in the Permian, and preserve many primitive characters. 

 They may have a precoracoid as well as a coracoid in the 

 shoulder girdle, and some even retain the cleithrum. The 

 most highly developed forms are the Theriodonts, which are 

 the ancestors of the mammals, and they foreshadow the 

 characters of the latter in many respects. The skull had two 

 occipital condyles, a false palate was present, and the teeth 

 were modified into incisors, canines, premolars and molars. 

 The dentary was large and beginning to take on the articulation 

 with the squamosal, while the quadrate became small and loose. 

 In the pelvic girdle the ilium showed the mammalian character 

 of pointing forwards, and the limbs were long and supported 

 the body clean off the ground. A typical Theriodont is Cyno- 

 gnathus, but it is probable that several of its characters were 

 evolved parallel with the mammals, having been derived from 

 a more primitive ancestor common to it and to the mammals. 



Among Synaptosauria, the Sauropterygia or Plesiosaurs 

 have become secondarily adapted to an aquatic mode of life. 

 They preserve primitive features such as the gastralia, which 

 are remnants of the ventral dermal bones or osteoscutes of the 

 Stegocephalian amphibia, but their limbs become modified 

 into paddles. This modification has not proceeded as far in 

 the Triassic Nothosaurus as in the Jurassic Plesiosaurus. The 

 Plesiosaurs reached lengths of 50 feet. 



In the next group or Parapsida, the roof of the skull was 

 perforated by a single superior temporal vacuity, above the 

 postorbital and squamosal bones. The hind border of the 

 vacuity is formed by a bone concerning the homology of which 

 doubt remains (see p. 104), but which may be the supra- 

 temporal. Here belong the Ichthyosaurs and the Squamata 

 (Lacertilia and Ophidia). 



The Ichthyosaurs are primitive in retaining the gastralia, 

 and a foramen for the pineal eye between the parietals, but 

 otherwise they are specialised in adaptation to an aquatic mode 

 of life. Median dorsal and tail-fins are developed, and the 

 limbs become modified into paddles, so much so that it is 

 impossible to determine the nature of the 5th metatarsal. A 

 series of progressive modification can be traced from the 



