 The pelvic fins are small and narrow . The thin , rapidly tapering tail is about equal to the disc in length and has an almost cylindrical cross @-@ section , without fin folds . The tail also has no venomous stinging spine , unlike other members of the family . A dense patch of flattened , heart @-@ shaped dermal denticles covers the center of the disc and extends onto the tail . Larger individuals additionally have numerous tall , sharp thorns over the entire upper surface of the disc . The porcupine ray is plain light to dark gray or brown above , darkening to blackish towards the tail tip , and white below . This large species grows to at least 1 @.@ 2 m ( 3 @.@ 9 ft ) across and 2 @.@ 2 m ( 7 @.@ 2 ft ) long , and may reach 1 @.@ 5 m ( 4 @.@ 9 ft ) across . 
