The close parallelism in outward form displayed by different genera and families of algae is in itself sufficient argument against the use of recent generic names for fossils of which the algal nature is often more than doubtful. Were external form to be accepted as a trustworthy guide, in the absence of internal structure and reproductive organs, such a genus as _Caulerpa_[225] would afford material for numerous generic designations. A comparison of the different species of this Siphoneous green alga brings out very clearly the exceedingly protean nature of this interesting genus, and serves as one instance among many of the small taxonomic value which can be attached to external configuration. _Caulerpa pusilla_ Mart. and Her., _C. taxifolia_ (Vahl.), _C. plumaris_ Forsk., _C. abies-marina_ J. Ag., _C. ericifolia_ (Turn.), _C. hypnoides_ (R. Br.), _C. cactoides_ (Turn.), _C. scalpelli formis_ (R. Br.), and others clearly illustrate the almost endless variety of form exhibited by the species of a single genus of algae. We constantly find in the several classes of plants a repetition of the same form either in the whole or in the separate members of the vegetative body, and but a slight acquaintance with plant types should lead us to use the test of external resemblance with the greatest possible caution. To emphasize this danger may seem merely the needless reiteration of a self-evident fact, but there is, perhaps, no source of error which has been more responsible for the creation of numerous worthless species among fossil plants. [Illustration: FIG. 30. 1. Rill-mark (after Williamson). 2. Trail made by a seaweed dragged along a soft plaster of Paris surface (after Nathorst). 3. Tracks made by _Goniada maculata_, a Polychaet (after Nathorst). 4. Burrow of an insect. 4_a_. Section of the gallery (after Zeiller).] There is, however, another category of impressions and casts of common occurrence in sedimentary rocks which requires a brief notice. Very many of the fossil algae described in text-books and palaeobotanical memoirs have been shown to be of animal origin, and to be merely the casts of tracks and burrows. A few examples will best serve to illustrate the identity of many of the fossils referred to algae with animal trails and with impressions produced by inorganic agency.