148 Prof. H. G. Seeley. The Shoulder Girdle 



Q 



Fig. 15. — Fore limb of Murcenosaurus (Cryptoclidus) platymera.s. 



separate tones to three. There are only three carpals in the second 

 row, though there may be a rudimentary fourth bone on the anterior 

 margin. 



The first digit is slender and short in this species, and all the phalang- 

 eal bones are sub -cylindrical, showing, as in other species of Murceno- 

 saurus no trace of the compression which characterised Cimoliosaurus. 



It is not improbable that, with fuller knowledge, the conceptions of 

 genera here indicated may, in some cases, be modified ; but, till better 

 examples of the American genera are found and figured, it will be 

 difficult to contrast them with those now described, and make the 

 definitions exact. 



IV. Classification. 



Characters of value in classification show gradations of develop- 

 ment in the Sauropterygia. This is conspicuous in the size and 

 form of the head, the relative length of the neck, the mode of articu- 

 lation of the cervical ribs by two heads or by one, or by anchylosis, 

 the length of the centrum in relation to its breadth in the several 

 regions of the vertebral column, the form and mode of attachment of 

 the neurapophyses, the form of the zygapophyses, the structure of 

 the shoulder girdle, the forms and conditions of the mesopodial 



