THE BONES IN" TKE ENALTOS.VURIA. 31.8 



The palate differs in being closed mesially. The vomers appear 

 to extend far forward between the maxillary bones. The median 

 bones behind the vomers consist of palatine and pterygoid with a 

 transverse bone external to these. In botb the occipital condyle 

 is single ; but in Plesiosaurs the exoccipital bones usually enter 

 into it. In both there is a foramen parietale. In Plesiosaurs 

 the quadrate bone is directed backward, as in crocodiles and Te- 

 ieosaurs, and is more intimately united to the skull than in Ich- 

 thyosaurs. The teeth differ in no essential, except that in Plesio- 

 saurs the fang is cylindrical and the crown has a tendency to curve 

 backward. 



The pectoral girdle has much in common m plan in the two 

 types, though the forms of the bones differ greatly. In order to 

 convert the lolithyosaurus into Plesiosaurus, it would be necessary 

 to amalgamate the clavicle and interclavicle into one bone, and 

 then contract the three arms till the scapulae were drawn almost 

 together in front, and the median ray only just met the coracoid 

 behind. Sometimes the interclavicle entirely disappears ; and then 

 the scapulae grow together mesially to replace it, and meet the 

 coracoid mesially. The coracoid bones would require to be rela- 

 tively enlarged and to be prolonged further backward. 



The pelvis of Plesiosaurus in none of its elements closely 

 resembles that of Ichthyosaurus. The ilium differs in being 

 straighter, more massively expanded at the femoral end, and usually 

 more compressed at the vertebral end. The ischium differs in 

 being directed backward, and in being usually more extended alono- 

 the symphysial line. The pubis is entirely different, being in Ple- 

 siosaurus subreniform. 



The vertebral column has nothing in common in the two groups, 

 except the biconcavity of the centrum. In Plesiosaurus the cen- 

 trum is more elongated from back to front, and the neural arch is 

 usually anchylosed to the centrum of the cervical vertebrae. 



The femur differs in the relatively larger size of the proximal 

 end, in wanting an inner trochanter at the proximal end, in the 

 greater elongation of the shaft, and in the greater expansion of 

 t he distal end. 



In Plesiosaurus there is some ^similarity in the ulna and ra- 

 dius, and carpus and tarsus, to those regions in Ichthyosaurus ; 

 and it is only by minute comparison that the^ bones can always 

 be distinguished. 



'2'.i* 



