The study of fossil animals embraces a much greater range of subject than that of fossil plants. The _fauna_ of any particular geological formation, that is to say, its embedded animal remains, for the most part vastly exceeds in number its _flora_, or vegetable remains, and is likewise usually better preserved. About the nature and affinities of several tribes of fossil plants there hangs an amount of uncertainty which renders them a dubious guide to the climatal and other conditions of the period and locality in which they lived. Generic distinctions among living plants often rest on the character of those parts which are the most perishable, such as flowers and seed-vessels. These delicate structures we, of course, can hardly look to find preserved in the rocks, and we have in place of them only detached leaflets, twigs, branches, and stems, often sorely mutilated in outward form, and presenting no trace of internal organization. But the tribes of the animal kingdom have, for the most part, harder frameworks. The minute infusoria, which by their accumulated remains help to choke up the delta of the Nile, and swarm by millions in every ocean of the globe, have their silicious or calcareous shells so minute that Ehrenberg has estimated a cubic inch of tripoli to contain forty-one thousand millions of them. The polypi have their internal calcareous skeletons, which abound in all the older limestones, and form the coral reefs of the present day. The mollusca, too, though, as their name imports, they have perishable bodies, are yet, in most cases, furnished with hard calcareous shells, that indicate by their various modifications of form and structure, the character of the animal that lived within them. They are found in all the formations from the earliest upwards, and as they vastly exceed in numbers all the other classes with which the geologist has to deal, they form the larger part of that basis of evidence from which he interprets the past history of organized existence. Hugh Miller loved to talk of them as the "shell alphabet," out of which the language of palæontological history should be compiled. The vertebrata, too, all have their hard skeletons, easily capable of preservation, whether it be in the form of the massive exo-skeleton of bone that characterized the older ganoidal fishes, or the compact endo-skeleton of the reptiles and mammals. A greater amount of attention is, therefore, due to the study of fossil animals, since they thus not only far exceed fossil plants in number, but possess a higher value as evidence of ancient physical conditions.