The collar-bones, or clavicles, are wanting in crocodiles; there is a slender interclavicle; and the shoulder-blade and coracoid are well developed (Fig. 102). The bones of the pelvis are loosely united with each other as they are in most reptiles (Fig. 104). The pubes, the anterior bones below, unlike those of all other reptiles, do not help to form the acetabulum or socket for the articulation of the thigh bone, nor do they articulate with each other. This single character sharply distinguishes a crocodile from all other reptiles, living or extinct, and is almost the only character that separates the order from the dinosaurs, aside from the peculiar structure of the nasal passages in the skull. On the under side of the body, connected with the front end of the pubes, there are seven or eight pairs of abdominal ribs, corresponding to the plastron of the turtles and similar to those of the ichthyosaurs and plesiosaurs. The mosasaurs have no such ribs. [Illustration: FIG. 102.] [Illustration: FIG. 103. FIG. 102.—Scapula (_sc_) and coracoid (_cor_) of gavial. FIG. 103.—Front foot of crocodile: _u_, ulna; _r_, radius; _re_, radiale; _ue_, ulnare; _p_, pisiform.] Furthermore, the crocodiles differ from all other living reptiles in having a four-chambered heart, like that of birds and mammals, that is, a heart with two auricles and two ventricles. This more perfect structure of the circulatory organs does not, however, insure at all times a complete separation of the pure or arterial blood from the impure or venous blood, since the blood may be more or less intermixed outside of the heart by a connection between the venous and the arterial systems. Whether these imperfectly developed organs, so suggestive of a higher and more perfect mode of respiration, are the vestiges of what were once among some reptiles functional structures, or whether they are rudiments of a higher organization, developing independently in these creatures, cannot be positively determined, but it seems very probable that, far back in geological times, some reptiles, especially the pterodactyls and dinosaurs, had their respiratory and circulatory systems more like those of the birds and mammals of today. Unfortunately, however, if such was the case, we may never be able to prove it, although proof would not be impossible; stranger things than fossil hearts have been found by paleontologists!