The attempt to discover the limits within which this remarkable exoskeleton is to be found, led me to look, more critically than I had previously done, into the arrangement and specific characterization of the recent _Crocodilia_. I have thereby arrived at results which, imperfect as they are, may be of service by leading others to inquire into the exact characters of species not at present within my reach; and I therefore propose to preface my account of the peculiarities of the exoskeleton in two of the genera of recent Crocodiles with some remarks on the classification of the group, and with a few notes upon the characters of the species and the limits of the genera. Everyone is acquainted with the great improvement effected in this branch of Herpetology by Cuvier, who divided the Crocodiles, which he regarded as constituting only a single genus, into the three subgenera _Alligatores_, _Crocodili_, and _Longirostres_. Subsequent writers have admitted these highly natural subdivisions; but there has been a constant tendency to raise their rank. The genus _Crocodilus_ has become the order _Crocodilia_; the subgenera _Alligatores_, &c., have been elevated into families; Dr. Gray has shown that the _Alligatores_ must be divided into three genera, and that there are at least two genera of _Crocodili_; and, while one of Cuvier's species of _Longirostres_ has been suppressed, the group is very generally retained with a changed name (_Gavialis_), a very important addition having been made to it in the _Crocodilus Schlegelii_ of Müller and Schlegel. Unless the considerable materials contained in the British Museum, the Hunterian collection, the collection of Dr. Grant, and the Christchurch Museum at Oxford had been freely placed at my disposal, I should have been wholly unable to acquire the information contained in the following pages. It is only right, therefore, that I should take this opportunity of offering my thanks to my friends Dr. Gray, Prof. Quekett, Dr. Grant, and Dr. Rolleston for the many facilities they have liberally afforded me. The recent species of the order _Crocodilia_ are divisible into three families, which correspond with the original subgenera of Cuvier, and may be termed the _Alligatoridæ_, the _Crocodilidæ_, and the _Gavialidæ_. I. In the ALLIGATORIDÆ the teeth are strong and unequal, and the posterior ones differ greatly in shape from the anterior. The anterior pair of mandibular teeth, and the fourth pair (or the so-called canines) are received into pits in the margins of the premaxilla and maxilla; while the mandibular teeth behind these pass inside, and not between, the maxillary teeth. The mandibular symphysis does not extend back beyond the level of the fifth tooth, and often not nearly so far. The line of the premaxillo-maxillary suture on the palate is straight, or convex forwards. The wide posterior nares look downwards, and are situated forwards on the palate.