In some cases we have I believe trustworthy examples of the Bacteria themselves, both in coprolites and plant-tissues, but it is more than probable that some of the recorded examples are not of any scientific value. The examination of petrified tissues under the higher powers of a microscope often reveals the existence of numerous spherical particles and rod-like bodies which agree in shape with _Micrococci_ or _Bacilli_. Minute crystals of mineral substances may occur in the siliceous or calcareous matrix of a petrified plant which simulate minute organic forms. Vogelsang[220] in his important work _die Krystalliten_ has thrown considerable light on the ontogeny of crystals, and the minute globulites and other forms of incipient crystallisation might well be mistaken for Bacterial cells. Granting, however, that we have satisfactory evidence, both direct and indirect, that some forms of Bacteria lived in the decaying tissues of Palaeozoic plants, and in the intestines of reptiles and other animals, we cannot safely proceed to specific diagnoses and determinations[221]. Renault has pointed out that fossil Bacteria may often be more readily detected than living forms owing to the presence of a brown ulmic substance which results from the carbonisation of the protoplasm. He is forced to admit, however, that such diagnostic characters as are obtained by Bacteriologists by means of cultures cannot be utilised when we are dealing with fossil examples! We are told that “Partout où nous avons cherché des Bacteriaceés, nous en avons rencontré.”[222] This indeed is the danger; an extended examination of fossil sections under an immersion-lens must almost inevitably lead to the discovery of minute bodies of a more or less spherical form which _might_ be _Micrococci_. To measure, and name such bodies as definite species of _Micrococci_ is, I believe, but wasted energy and an attempt to compass the impossible. Specialists tell us that the accurate determination of species of recent Bacteria is practically hopeless: may we not reasonably conclude that the attempt to specifically diagnose fossil forms is absolutely hopeless? “The imagination of man is naturally sublime, delighted with whatever is remote and extraordinary—”, but it is to be deplored if the fascination of fossil bacteriology is allowed to warp sound scientific sense. IV. ALGAE. A. DIATOMACEAE. (Diatoms.) B. CHLOROPHYCEAE. (Green algae.) C. RHODOPHYCEAE. (Red algae.) D. PHAEOPHYCEAE. (Brown algae.) The presence of chlorophyll is one common characteristic of the numerous plants included in the Algae. The generally adopted classification rests in part on an artificial distinction, namely the prevailing colour of the plant.