Group - Reptilia ; Characteristics - Crawling mode of locomotion, all are terrestrial
Group - Chondrichthyes ; Characteristics - Placoid scales, electric organs, claspers
Group - Amphibia ; Characteristics - Tympanum, two pairs of limbs
Group - Mammalia ; Characteristics - Mammary glands, lungs, direct development
All mammals possess mammary glands, lungs and undergo direct development. Some reptiles are aquatic (e.g., sea snakes, turtles). Electric organs are present only in some cartilaginous fishes (e.g., Torpedo); not all. Some amphibians are limbless (e.g., Ichthyophis).