Crocodiles (Order Crocodilia) are the largest living reptiles and possibly the most advanced, having a nearly complete four-chambered heart. The nostrils are at the end of the snout and the eyes protrude from the head so that these animals can float near the surface of water with only these parts exposed above the water. It was possibly these features that allowed them to survive the sudden global cooling that almost definitely occurred at the end of the Cretaceous period. Under hot conditions crocodiles open their mouths and air passes over the soft skin on the inside of the mouth and cools the animal down. The crocodile eyes are unusual in that the photo- pigments receptive to light are different in the upper and lower hemispheres of each retina. The upper retinal hemisphere which looks down into the water has a photopigment similar to that of freshwater animals (porphyropsin), whereas the ventralretinalhemisphere has the pigment of terrestrial animals (rhodopsin). The skin is thick and covered with horny epidermal scales and dorsal bony plates (osteoderms) which may extend to the ventral surface and are like those in turtle shells.