The name was proposed by Sir Joseph Hooker for some specimens found by Dr Strickland[390] in the Ludlow bone-bed (Silurian) of Woolhope and May-Hill. Examples were subsequently recorded from the Wenlock limestone of Malvern and from Silurian and Old Red Sandstone rocks of other districts. Hicks[391] found _Pachytheca_ in the Pen-y-Glog grits of Corwen in association with _Nematophycus_, and the two fossils have been found together elsewhere. This association led to the suggestion that _Pachytheca_ might be the sporangium of _Nematophycus_, and Dawson[392], in conformity with his belief in the coniferous character of the latter plant, referred to _Pachytheca_ as a true seed. 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._