This family embraces three genera, readily distinguishable by osteological characters--_Alligator_, _Caiman_, and _Jacare_. Genus 1. ALLIGATOR. Dental formula, 20-20/20-20. 9th maxillary tooth the largest of its series. The snout is very broad, flattened, and rounded at the end. There is an indistinct longitudinal interorbital ridge; and there are two short ridges along the line of junction of the prefrontal and lachrymal bones. The aperture of the external nares is divided into two parts, by the prolongation forwards of the nasal bones. The supra-temporal fossæ are well-marked and open, though not large. The vomers do not appear in the palate. The feet are well webbed. The dorsal bony scutes are not articulated together; and there are no ventral scutes. This genus contains only one species, the well-known _Alligator Mississipiensis_, or _lucius_, which is exclusively North American. Cuvier (Oss. Foss. ed. 4. vol. ix. p. 211) gives the appearance of the vomer in the palate as a general character of the _Alligatores_; but this bone is not visible in the palate of any of those _Alligatores_ which Cuvier would have referred to his _A. lucius_ or _A. palpebrosus_, and which form the genera _Alligator_ and _Caiman_ as here defined. The vomers are in fact as slender and delicate as in the Crocodile, and extend only between the level of the tenth maxillary tooth anteriorly and the descending processes of the prefrontal posteriorly. What may be called the median nares, or the arch formed by the postero-lateral part of the vomer and the anterior and superior lamina of the palatine bone on each side (which would constitute the posterior boundary of the posterior nares, if the palatine and pterygoid bones gave off no inferior or palatine processes), are situated nearly on a level with the twelfth tooth, or with the palato-maxillary suture. Genus 2. CAIMAN. Dental formula 20-20/22-22 (Natterer). The face is without median or transverse ridges, but it is sharply angulated along a line which extends from the orbit forwards along the sides of the snout. The anterior nasal aperture is undivided in the dry skull. The vomers do not appear in the palate. The supra-temporal fossæ are obliterated, the circumjacent bones uniting over them. The webs of the feet are rudimentary. The dorsal scutes are articulated together by lateral sutures and anterior and posterior facets; and there is a ventral shield, consisting of similarly articulated scutes. Natterer[1] has described three species of _Caiman_--_C. palpebrosus_, _C. trigonatus_, and _C. gibbiceps_. The Caimans abound chiefly in tropical South America; but they are found as far north as Mexico, a specimen of _C. palpebrosus_ in Dr. Grant's collection coming from that country. Genus 3. JACARE.