32G PROF. Xi. a. SEELEY ON STMTLTTUDES OF 



In the limbs there is nothing in common. 



§ 6. The Rhynclioceplialoid Oliaracters of Plesiosaurus. 



The skull of Satteria has much such an outline as that of Ple- 

 siosaurus dolichodeirus ; and in the parietal and frontal regions 

 there is a similarity of form ; but in Satteria the postorbital bone 

 meets the squamosal, and forms an upper bar to the side of the 

 temporal fossa, which does not occur in Plesiosaurus. The orbits 

 in Satteria are larger and more vertical than in Plesiosaurus, 

 while the nares are lateral, and the premaxillary bones are deve- 

 loped anterior to them only to a small extent ; the skull also appears 

 to differ from that of Plesiosaurus in having all the median roof- 

 bones double. The under surface of the palate differs from that of 

 Plesiosaurus in not being closed in the median line, in not including 

 divided nares at the back of the palate, in the large develop- 

 ment of the pterygoid bones, which do not cover the basisphe- 

 nold, but extend along the inner wing of the quadrate bone, 

 and extend forward to meet the vomers, while the palatine bone 

 occupies the same lateral position between the pterygoid and 

 maxillary which would entitle it to be considered homologous 

 with the transverse bone in Plesiosaurus. The teeth are utterly 

 unlike those of Plesiosaurus in form, in being blended with the 

 jaw, in being also carried on the palatine bones. 



The vertebral column differs from Plesiosaurus in the fewness 

 of the vertebrte ; though each vertebra agrees in having a not 

 dissimilar form and a biconcave centrum. The atlas and axis are 

 not like those of Plesiosaurus, the latter bone having an extended 

 neural spine and an odontoid process. The fourth vertebra has 

 a divided attachment for the ribs such as is sometimes seen in the 

 cervical vertebrae of Plesiosaurs ; but the cervical ribs have not 

 the hatchet-shape. In Satteria the intervertebral wedge bones 

 are continued down the neck, which does not happen in Plesio- 

 saurus. It has seemed to me to be not improbable that the con- 

 version of amphicoelous vertebrae into procoelous or opisthocoelous 

 vertebrae has been determined by the anchylosis of the inter- 

 vertebral wedge bones to the anterior or posterior margin of the 

 centrum. In the dorsal region the ribs ai-e supported by single 

 heads, as in Plesiosaurus, but not from transverse processes. The 

 caudal vertebrae carry chevron bones : the centrum differs from that 

 of Plesiosaurus in being ossified in two parts. The cervical and 



