They are kept in the same class i.e class reptile because they share common features which are as follows:
1) Both are cold blooded.
2) They lay eggs.
3)They have scales on their body.
4)Three chambered heart
5) Respiration through lungs.
we keep snake turtle in same class because - both lay egg with tough covering outside water -they have dry skin with scales -they are triploblastic -they are bilateraly symmetrical -both are cold blooded animals with 3 chambered heart