1. Amphibians 2. Reptiles 3. Birds 4. Mammals a. intrinsic musculature is reduced; pectoralis (downstroke muscle) and supracoracoideus (upstroke muscle) are enlarged b. similar to reptiles but more diverse c. much more complex than in fish d. more numerous and diverse than in amphibians; better support of body and increased mobility of distal segments of the limbs