a croc question

[Rodlox R <rodlox@hotmail.com>]

got a question...


on http://www.fugglefamily.com/NaturalHistory3.htm it says "Goniopholids had 
strongly built, low-profiled skulls, with ornamented bone surfaces and 
patterned body armour, thus differing from the metriorhynchids. Like their 
marine cousins, however they had nearly flat-faced centra to their 
vertebrae, which contrasts with the socket-and-ball development found in the 
more modern forms."


what are "centra" ?


thanks.