47 6 PLESIOSAURIA 



shortened radius and ulna, tibia and fibula; but the phalanges, 

 which increase to about ten, are always longer than broad, and 

 there is no indication of an increase of the number of fingers, or 

 of additional, lateral, phalanx-like nodules. The pelvis is very- 

 strong ; the broad pubes and ischia meet in the middle line, and 

 they either enclose one wide undivided foramen, or the two 

 symphysial portions meet, and there are then two obturator- 

 foramina. The pubes are generally much larger, especially 

 broader, than the ischia ; and although partaking in the 

 formation of the acetabulum, they do not articulate with the ilia, 

 at least not in Plesiosauri. The ilia are always small ; in 

 Plesiosauri attached to only one or two sacral ribs ; to three or 

 four in the Triassic genera. 



Ichthyosauri and Plesiosauri were combined as " Enaliosauria " 

 by Conybeare. Owen recognised their fundamental differences, 

 and separated them as " Ichthyopterygia " and " Sauropterygia," 

 according to the structure of the limbs. We now know that 

 the paddles of the Ichthyosauri bear but a superficial resemblance 

 to the fins of fishes, and are fundamentally referable to the 

 pentadactyle type, as are the paddles of the Plesiosauri, 

 although tlie latter retain more of the typical features of 

 reptilian limbs. It was soon recognised that the Nothosauri 

 are allied to the Plesiosauri, but the Mesosauri (until then 

 vaguely grouped with the Ehynchocephalia, or linked with 

 Protorosauri as Proganosauria) have only recently^ received their 

 proper place in the system as members of the Plesiosauria, 

 which we divide into two main groups. , 



Order I. NOTHOSAURI' 



The limbs are of the terrestrial type ; the five digits have 

 the usual number of phalanges, which do not exceed five. The 

 bones of the limbs are slender ; the humerus has an entepi- 

 condylar foramen. 



Fam. 1. Mesosauridae. — The neck contains about ten 

 vertebrae. The vertebrae are deeply biconcave, perforated by the 

 chorda dorsalis. Sacral vertebrae four in number. Clavicles strong; 

 interclavicle very small. Mesosaurus, the only genus, with one 

 species, M. tenuidens, about one foot in length, was found in 

 ' Boulenger, Trans. Zool. Soc. xiv. 1898 (read Nov. 1893). ' 1^6^05 = si>ui-ioiis. 



