821 PlIOF. ir. G. SEELEY ON SIMILITUDES OF 



same plane with the pubis, being directed somewhat down- 

 ward. 



As with the fore limb, so here the pubis might be supposed to 

 combine with the hyposternal, the ischium with the xiphosternals. 

 The plesiosaurian limbs are only comparable with the marine type. 

 The chelonian femur differs chiefly in being shorter, less expanded 

 distally in antero-posterior extent, and in having a large trochanter 

 behind the proximal articulation. The humerus is somewhat 

 similar distally, but not enough expanded ; the bone is too short, 

 and the trochanters at the proximal end are dissimilar. The 

 tibia, fibula, ulna, and radius in chelonians are elongated, and 

 so are incomparable. The carpal and tarsal bones are not dis- 

 similar individually ; but their arrangement is not like that in 

 Plesiosaurus. The phalanges are not dissimilar in form, except 

 that in Ghelone the bones are vastly more elongated and much less 

 numerous ; but the hour-glass shape and flattened articular ends 

 are similar. 



§ 5. The Lacertian Characters of Plesiosaurus. 



The general outline of a lizard's skull is the same as that of 

 Plesiosaurus; and in such a lizard as Iguana the orbits and 

 temporal fossae are similarly placed. But in Plesiosaurus the 

 nares are further back and smaller ; and thus the nasal bones 

 are narrowed to extend between them, and the premaxillary 

 bones enlarge to form the prenasal part of the skull. In Iguana 

 the postfrontal is divided, but the superorbital part does not ex- 

 clude the frontal bone from the orbit as in Plesiosaurus ; but the 

 back of the orbit is similarly formed by its postorbital part. In 

 lizards the maxillary bone is not admitted into the base of the 

 orbit as it is in Plesiosaurus. The parietal is similarly compressed 

 into a longitudinal ridge, and similarly sends off processes behind 

 which diverge outward and backward. The processes in Plesio- 

 saurus appear to be overlapped by the squamosal bones, while in 

 lizards the squamosal bones are overlapped by them. In lizards 

 the quadrate bone is naked at the sides ; but in Plesiosaurus it is 

 covered by the squamosal and quadrato-jugal. And in Plesiosaurs 

 the occipital segment of the skull is not prolonged beyond the 

 parietal segment. 



The teeth differ from those of lizards in being placed in sockets 

 and in the series being similar from the back of the jaw to tlie 

 front. 



