XI DOLICHOSAURI MOSASAURI 489 



meeting also in the middle line. The ilia are loosely attached 

 to two vertebrae in the Dolichosauri ; in the Mosasauri they 

 have lost this connexion. Both anterior and posterior limbs 

 are transformed into pentadactyle paddles, with much shortened 

 and broadened bones of the arms and legs. The digits are to 

 a certain extent hyperphalangeal, since several of them possess 

 five phalanges. 



The Pythonomorpha are undoubtedly allied to the Sauria, 

 but they are certainly not their ancestors, since typical Autosauri 

 occur in the Lower Chalk ; nor are the Snakes their descendants, 

 in spite of many convergent resemblances. We consider them 

 to be the marine collateral branch of the Sauria, which rapidly 

 developed highly specialised, often very large forms, restricted to 

 the Cretaceous epoch, with a wide, cosmopolitan distribution. 



Order I. DOLICHOSAURI. 



This older group is characterised by the sutural symphysial 

 connexion of the two mandibles and by the possession of two 

 sacral vertebrae. The body is snake-like. Pleurodont. Doli- 

 cliosaurus longicollis of the Lower Chalk of Kent and Sussex ; 

 total length about 3 feet, with about seventeen cervical vertebrae 

 and pleurodont teeth. Acteosaurus of Istria ; anterior extremities 

 distinctly shorter than the posterior pair ; tail long. Vertebrae, 

 like those of Dolichosaurios, with zygosphenes. Plioj)lateca7']pus 

 of the Upper Chalk of Holland has a slender interclavicle ; the 

 vertebrae are without zygosphenes, but those of the cervical 

 region possess a downwardly directed long hypapophysial process 

 with a separately ossified epiphysis. 



Order II. MOSASAURI. 



The two halves of the lower jaw are connected by ligament 

 and are therefore movable as in Snakes. There are no sacral 

 vertebrae, the pelvis having lost its connexion with the vertebral 

 column. The formation of the limbs into paddles is more pro- 

 nounced than in the Dolichosauri. 



Mosasaurus, the chief genus, so called from Mosa, the Latin 

 name of the river Maas, with several species from the Upper 

 Cretaceous strata of the ^Netherlands, England, and North 



