230 PALEONTOLOGY 



ribs and sternum may perhaps be discerned a first step iu 

 that series of development of the lmemapophyses of the trunk 

 which reaches its maximum in the plastron of the Chelonia. 



The connation of the clavicle with the scapula is common 

 to the Chelonia with the Plesiosauri ; the expansion of the 

 coracoicls — extreme in Plesiosauri — is greater in Chelonia 

 than in Crocodilia, but is still greater in some Lacertia. The 

 form and proportions of the pubis and ischium, as compared 

 with the ilium, in the pelvic arch of the Plesiosauri, find the 

 nearest approach in the pelvis of marine Chelonia; and no 

 other existing reptile now offers so near, although it be so 

 remote, a resemblance to the structure of the paddles of the 

 Plesiosaur. Amongst the many figurative illustrations of the 

 nature of the Plesiosaur in which popular writers have 

 indulged, that which compares it to a snake threaded through 

 the trunk of a turtle is the most striking ; but the number of 

 vertebrae in the Plesiosaur is no true indication of affinity with 

 the ophidian order of reptiles. 



The reptilian skull from formations underlying the lias, to 

 which that of Plesiosaurus has the nearest resemblance, is the 

 skull of the Pistosaurus ; in this genus the nostrils have a 

 similar position and diminutive size, but are somewhat more 

 in advance of the orbits, and the premaxillaries enter into 

 the formation of their boundary : the premaxillary muzzle and 

 the temporal fossae are also somewhat longer and narrower. 

 The post-frontals and mastoids more clearly combine with 

 malars and squamosals in forming the zygomatic arch, which 

 is of greater depth in Pistosaurus; the parietal foramen is 

 larger; there is no trace of a median parietal crest. On the 

 palate, besides the vacuities between the pterygoids and pre- 

 sphenoids, and the small foramina between the palatines, pre- 

 maxillaries, and nia.xillaries, there is in Pistosaurus a single 

 median foramen in advance of the Latter foramina, between 

 the pointed anterior ends of the pterygoids and the premaxil- 



