Incompletely divided ventricle, and no more of the spiral valves seen in amphibians. Lungs work by negative pressure in reptiles, in contrast to amphibians. The diaphragm as a muscle of respiration is seen only in mammals. Most reptiles have Jacobson's organs as accessory olfactory organs. Water balance .
Reptiles excrete uric acid , which is semisolid and nontoxic.
Reptiles are well represented by the fossil record. A major way of distinguishing the major groups of reptiles is through skull modifications for jaw muscle attachment. Early enlargement of reptilian jaw musculature, e.g, the temporalis muscle for crunching. We use this as the major basis for classification, although recently some zoologists have viewed turtles as more in common with diapsids and not as primitive anapsids. No teeth, but a horny beak.