and Clavicular Arch in Sauropterygia. 



137 



given. In it the ilium articulates with the ischium only, as in some 

 species of Murmnosaurus from the English Oxford Clay (Leeds Collec- 

 tion, Brit. Mus.). The shoulder girdle is on the Elasmosaurian plan, 

 enclosing two vacuities, but the structure of the glenoid cavity is 

 distinctive. There are three bones in linear succession at the distal 

 end of the humerus and femur. The tibia and fibula are transversely 

 extended, and of oblong form, apparently resembling Colymbosaurus. 

 The phalanges are unusually numerous. The neural arch is anchy- 

 losed to the centrum. The neural canal is large. The cervical 

 vertebras are sub-quadrate, depressed, and transversely wide. The 

 dorsal centrum is sab-circular. The articular faces are shallow 

 concavities. 



The characters assigned to Polycotylus, Cimoliosaurus, Elasmo- 

 saurus, and Trinacromerum are such as enable the types to be recog- 

 nised ; and, therefore, pending fuller information, it is convenient to 

 adopt them as genera limited, so far as is at present known, to the 

 Cretaceous period. It is probable that all belong to the Elasmo- 

 sauridse, but Elasmosaurus and Trinacromerum are the only types in 

 which the shoulder girdle is known. The oblong form of the tibia in 

 Trinacromerum and Polycotylus, and the transverse elongation of a 

 carpal in Cimoliosaurus, make it probable that the middle segments 

 of the limbs had the bones transversely elongated in all these genera. 

 In none of them have clavicles as yet been recognised. 



Polyptyclwdon is probably to be included with these genera ; but it 

 is only known from teeth, cranial fragments, and vertebral centra, 

 which do not differentiate the genus ; though the cervical vertebras 

 (' Quart. Journ. Geol. Soc.,' vol. 32, p. 433) are relatively short and 

 deep. 



The Elasmosauridaa are well represented in the Cretaceous rocks of 

 this country. Two genera, Murainosaurus and Colymbosaurus, have 

 also been regarded as peculiar to the Oxford and Kimeridge Clays. 

 These genera are best defined by the bones of the extremities. In 

 both the bones of the shoulder girdle are essentially the same. 



In Murainosaurus the cervical region is long. The zygapophysial 

 facets have a cylindroid curve. The articular faces of the centra are 

 rather wider than deep, though nearly circular and biconcave. The 

 ulna and radius are sub-quadrate. There is no third bone in the 

 fore-arm. The phalanges are stout and but little compressed. The 

 shoulder-girdle is on the Elasmosaurian type, with clavicles. The 

 type species is M. Leedsii* 



In Colymbosaurus the neck is equally long. The neural arch and 

 ribs are anchylosed to the centrum. The neurapophyses are lamellar 

 and compressed from side to side. The centrum is biconcave, but the 

 concavity decreases posteriorly. The articular surface is transversely, 

 * ' Geol. Soc. Quart. Journ.,' 1874, p. 197. 



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