Both spellings Kronosaurus and Kronosaur are present in the book. The former occurs more often.
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The Kronosaurus was a type of large pliosaur. They were massive creatures, some reaching 80 feet in length, with powerful, armoured heads.
The kronosaurus was one of the species that the Silurians brought back from extinction after they took over in a parallel world. Kronosaurus would frequently bother the remaining human submarines. When the HMS Revenge attempted to attack the Silurian base, they telepathically controlled hundreds of kronosauruses to deter them, ramming the submarine to damage it. (PROSE: Blood Heat)