474 PLESIOSAURIA 



right up to the vomers, separating the palatines from each other 

 thereby. Palatal vacuities are absent in Xuthosaurus ; small 

 and oval, between the palatines, pterygoids, ectopterygoids and 

 maxillaries in Lariosaurus ; still smaller in the Plesiosauri. 



The vertebrae are mostly biconcave, in the Triassic genera 

 stilt perforated by the chorda, while in many Plesiosauri the 

 centra are solid, with almost plane articulating surfaces. The 

 neural arches are usually firmly sutured, or quite fused with the 

 centra. Intercentra are absent, except as chevrons in the tail. 

 Although the cervical and some of the thoracic ribs of the 

 Triassic genera have typical capitula and tubercula, they articu- 

 late exclusively upon the centra, and not upon the neural arches 

 also. The number of cervical vertebrae amounts to nine in 

 Mesosaurus ; in Lariosaurus it is increased to about twenty ; and 

 in some Plesiosauri to between thirty and forty. The cervical ribs 

 are very short, but they increase gradually towards the thorax, 

 which is well protected by long and strong ribs, which decrease 

 again very gradually, being still long in the lumbar region. 

 There is, properly speaking, no sacrum, because the one to four 

 sacral ribs remain quite separate. The tail is still long in 

 Zariosaurus, consisting of about forty mucli shortened vertebrae ; 

 considerably shorter than the neck in most of the Plesiosauri. 

 A sternum is absent, but the belly is protected by many strong- 

 abdominal riljs, crowded together, and consisting each of a median 

 and two pairs of lateral pieces. 



The shoulder-girdle is very strong, composed of scapulae, 

 very strong coracoids, clavicles, and an interclavicle. The pre- 

 coracoids are indicated by a process and a notch in the Triassic 

 genera ; in the later forms they are abolished. The coracoids 

 always meet in the median line, and often produce a strong 

 symphysis. The scapulae possess a very prominent and large 

 acromial process, upon which rest the dorsal or lateral ends of 

 the clavicles. In some Plesiosauri tlie shoulder-girdle has under- 

 gone an absolutely unique modification. The correct interpre- 

 tation has been given by C. W. Andrews after the exannnation 

 of exquisitely preserved specimens of Cryptoclidus from the 

 Oxford clay of the Middle Oolite, near Peterborough. The 

 dorsal portion or main shaft of the scapula is reduced to what 

 now looks like a dorso-lateral process, while the broad acromial 

 process is much elongated, and lies in Flesiosaurus upon the 



