I have found this matter for the first time that Crocodiles are not a reptile. Crocodile such as birds and mammals are a separate class, which have been derived from reptiles. Mammalian four-chambered heart from a reptile-like three-chambered heart is extension of the septum (the wall dividing the chambers, lost of the right systemic arch and the left systemic arch persists). In birds, lost of the left systemic arch and the right systemic arch persist. The lineage leading to crocodiles evolved a four-chambered heart along a different pathway, keeping both systemic arches. Zoologists putting the land vertebrates in 4 classes (amphibians, reptiles, birds and mammals). But I came to the conclusion that their crocodile are a separate class. As birds and mammals, with obtaining, four-chambered heart, have been derived from the class of reptiles and create a separate class. Crocodile also, have made four-chambered hearts and create a new class and has been derived from the reptile class. Mammalian four-chambered heart from a reptile-like three-chambered heart is extension of the septum (the wall dividing the chambers, lost of the right systemic arch and the left systemic arch persists).