I have examined _Jacare fissipes_ and _nigra_, _Caiman trigonatus_, and _C. gibbiceps_, in the British Museum; and I find, in all, dorsal and ventral armour having the same essential arrangement as that just described. A specimen of _Caiman palpebrosus_ about two feet long, the opportunity of examining which I owe to Dr. Grant, exhibits the dorsal and ventral shields (whose scutes are in the main similarly arranged) very beautifully; and a young _Jacare_ of about 18 inches in length, for which I am indebted to the kindness of the same gentleman, proves that the scutes are developed even in specimens of this age. I have no hesitation therefore in expressing my belief that this singularly complete dermal armour will be found to be characteristic of all the species of the genera _Caiman_ and _Jacare_. On the other hand, I have examined _Alligator Mississipiensis_, _Crocodilus vulgaris_, _C. biporcatus_, _C. Americanus_, _C. rhombifer_, and _C. bombifrons_, _Mecistops cataphractus_, and _Gavialis Gangeticus_, of various ages and sizes, without having been able to discover a trace of ventral scutes. This is the more remarkable, as the well-marked ventral and dorsal shields of many of the ancient _Teleosauria_ would lead one to expect a corresponding exoskeleton (if anywhere) in their nearest allies, the modern _Gavialidæ_. However, _Goniopholis_, with its strong armour, is more like an ordinary Crocodile; and I have recently discovered that a true Crocodile in some respects curiously similar to _C. bombifrons_ (_C. Hastingsiæ_) was covered with scutes exceedingly like those of the modern _Caiman_ and _Jacare_. In minute structure the bony scutes of _Jacare_ closely resemble those of such a fish as a Sturgeon: a middle layer, containing so many canals as to appear almost cancellated in longitudinal or transverse section, is covered externally by a thin, and internally by a thick, layer composed of bony lamellæ, nearly parallel to the plane of the scute. Round the canals of the middle layer, the bony lamellæ are disposed concentrically, to a greater or less extent. The lacunæ are of very various shapes; and there are perhaps as many short as elongated forms. The canals of the middle layer communicate by large branches with the inner, by smaller and fewer branches with the outer surface of the scute.