The best sections of this fossil have been prepared with remarkable skill by Mr Storrie of Cardiff; they were carefully examined and described by Barber in two memoirs[393] published in the Annals of Botany, the account being illustrated by several well executed drawings and microphotographs.
Among other difficulties to contend against in the interpretation of Pachytheca there is that of mineralisation. The preservation is such as to render the discrimination of original structure as distinct from structural features of secondary origin, consequent on a particular manner of crystallisation of the siliceous material, a matter of considerable difficulty.
Suggestions as to the nature of Pachytheca have been particularly numerous; it has been referred to most classes of plants and relegated by some writers to the animal kingdom. The most recent addition to our knowledge of this problematic fossil was the discovery of a specimen by Mr Storrie in which the Pachytheca sphere rested in a small cup, like an acorn fruit in its cupule. This specimen was figured and described by Mr George Murray[394] in 1895; he expresses the opinion that the discovery makes the taxonomic position of the genus still more obscure. Solms-Laubach briefly refers to Pachytheca in connection with Nematophycus, and regards its precise nature almost as much an unsolved riddle now as it was when first discovered. For a fuller account of this fossil reference must be made to the contributions of Hooker[395], Barber[396] and others. The literature is quoted by Barber and more recently by Solms-Laubach[397]. There are several specimens and microscopic sections of Pachytheca in the geological and botanical departments of the British Museum. The genus has been recorded from Shropshire, North Wales, Malvern, Herefordshire, Perthshire and other British localities, as well as from Canada; it occurs in both Silurian and Old Red Sandstone rocks.
Algites.
A generic name for those fossils which in all probability belong to the class Algae, but which, by reason of the absence of reproductive organs, internal structure, or characters of a trustworthy nature in the determination of affinity, cannot be referred with any degree of certainty to a particular recent genus or family.
This term was suggested in 1894[398] as a provisional and comprehensive designation under which might be included such impressions or casts as might reasonably be referred to Algae. The practice of applying to alga-like fossils names suggestive of a definite alliance with recent genera is as a rule unsound. It would simplify nomenclature, and avoid the multiplication of generic names, if the term Algites were applied to such algal fossils from rocks of various ages as afford no trustworthy data by which their family or generic affinity can be established.
V. MYXOMYCETES (MYCETOZOA).
This class of organisms affords an interesting example of the impossibility of maintaining a hard and fast line between the animal and plant kingdom. Zoologists and Botanists usually include the Myxomycetes[399] in the text-books of their respective subjects, and the name Animal-fungi which has been applied to these organisms expresses their dual relationship. They constitute one of three groups which we may include in that intermediate zone or ‘buffer-state’ between the two kingdoms. From a palaeobotanical point of view the Myxomycetes are of little interest, but a very brief reference may be made to them rather for the sake of avoiding unnecessary incompleteness in our classification than from their importance as possible fossils.
They are organisms without chlorophyll, consisting of a naked mass of protoplasm, known as the plasmodium, which may attain a size of several inches. Such plasmodia creep over the surface of decaying organic substrata, and in forming their asexual reproductive cells they are converted into somewhat complex fruits containing spores. The spores produce motile swarm-cells, which eventually coalesce together to form a new plasmodium.
A few examples of fossil Myxomycetes have been recorded from the Palaeozoic and more recent formations, but none of them are entirely beyond suspicion. We may mention three examples of fossils referred to this group, but only one of these is entitled to serious consideration.