On what grounds can the class of reptiles be divided?

Reptiles can be divided into groups according to characteristics associated with their external and internal structure.
1) body integuments: if lizards and snakes have small horny scales, then in turtles it is a bone-horny carapace fused from the abdominal and dorsal shields, and in crocodiles – a combination of large horny shields with underlying bony plates;
2) the structure of the limbs: snakes do not have them, lizards have typical five-toed ones, in sea turtles they are transformed into flippers, and crocodiles have swimming membranes between the fingers;
3) from the features of the internal structure: in lizards, the chest and girdles of the limbs are well developed and there are both lungs, in snakes everything is the opposite (the ribs end freely), in turtles the skeleton is a single whole with the shell, and crocodiles, as the most advanced group, have lungs complex structure and heart with 4 chambers.