Numerous fossil species have been placed in genera of living coralline algae, but some of the placement is in doubt because of the fragmentary nature of the material. Much of the work on fossils has been concerned with describing plants that grew in different geographic areas during different geological ages. The placement of fossil taxa in extant genera must be examined with caution. The imperfect nature of fossils has caused difficulties in assigning material in genus. Four families of calcareous red algae containing fossils are presently generally accepted, they are as follows: Solenoporaceae, Gymnocodiaceae, Squamariaceae, and Corallinaceae. The most advanced types of coralline algae – the articulated forms – are also present in the fossilized state, being particularly common in the Tertiary. Studies of the structure and taxonomy of coralline algae have evolved along two lines: (1) studies of fossil coralline algae by paleobotanists and (2) studies of living plants that were fixed while fresh.