Another useful sources of freshwater turtles:
1. Species Identification of Wildlife Traded Commonly in South East Asia, produced by Traffic.
2. Turtles, terrapins and tortoises at BBC Nature Wildlife
Turtles, terrapins and tortoises are all in one order, called the Testudines. They all have a shell, though only certain species can withdraw their lead and limbs into it. All the many types of turtle, terrapin and tortoise lay eggs. In some, the temperature at which the egg incubated will decide whether the embryo becomes male or female. Further reading at BBC Nature Wildlife
3. Science Daily
Turtles are reptiles of the order Testudines (all living turtles belong to the crown group Chelonia), most of whose body is shielded by a special bony or cartilaginous shell developed from their ribs. Further reading at ScienceDaily
4. How to determine sex in chelonian
This information is important to know even for those not intending to breed their animals. A habitat that would be considered acceptable for two females may not be large enough for a pair and a habitat large enough for pair may not be large enough for two males. The sexual makeup of groups of animal also has a direct realtion to the amount of stress through aggression or mating attempts being experinced by some of the members of the group regardless of habitat size. This stress can have an impact on the overall health of an animal or group as a whole.
Further info at World Chelonian Trust