INTRODUCTION, ix 



hyperphalangy. Plastron consisting of ventral ribs arranged in rows, each consisting 

 of a median element and two or more lateral pairs. 



The primitive Therapsids seem to possess many of the above characters. Thus 

 in the skull of the Therocephalia we find a single temporal fossa (the zygomatic 

 arch being formed by the postorbital and squamosal), a large pineal foramen, 

 a fixed quadrate ; a postfrontal excluded from the temporal fossa, a prefrontal, a 

 lachrymal, and a septo-maxillary. The palate also is very similarly constituted, 

 and there is a parasphenoid (vomer) which in the later Theriodonts becomes large. 

 The teeth in the premaxillae and maxillae are thecodont, but are here differentiated 

 into incisors, canines, and cheek-teeth, a character which shows that this group 

 is too highly specialised to be the actual ancestor of the Sauropterygia. In 

 some forms there are small teeth on the pterygoids. In the shoulder-girdle the 

 precoracoid is ossified, but the reduction of this element to cartilage in the early 

 Sauropterygia is probably due to their beginning to be adapted to an aquatic life, 

 and to the development of their clavicular arch. The pubes and ischia, though 

 plate-like in the Therocephalia, become modified in the Cynodonts, and a similar 

 modification leading to the development of a large obturator foramen might have 

 given rise to the form seen in Nothosaurus. The large plate-like pubes and ischia 

 in the later Sauropterygia are, no doubt, as already noted, secondarily acquired. 

 The phalangeal formula in Lariosaurus (2, 3, 4, 4, 3 in the manus, 2, 3, 4, 5,4 in the 

 pes) might easily be derived from the Therocephalian 2, 3, 4, 5, 3 and 2, 3, 4, 5, 4. 



The great development of the ventral ribs in the Sauropterygia might be regarded a 

 difficulty in the way of regarding them as related to the Therocephalian stock, but this 

 is discounted by the fact that in the Dromasauria (Galeptcs and GalecMrus) ventral 

 ribs occur. This group, though in many ways specialised, is clearly closely related to 

 the primitive stock of the Therocephalians, in which, no doubt, ventral ribs and an 

 undifferentiated dentition existed. It is from some such early type as this, rather 

 than from any of the known Therocephalia, that the Sauropterygia may have arisen. 



The Crocodilia of the Oxford Clay were probably all aquatic to a much greater degree 



than the modern representatives of the order, but the various groups differ considerably 



among themselves as to the extent to which the skeleton has become modified for 



pelagic life. The Teleosauridar, represented by the genera Steneosaurus and 

 PART II. b 



