As Carruthers first pointed out the unseptate nature of the elements and the occurrence of large and small tubes forming a comparatively lax tissue suggested affinities with such recent genera as _Penicillus_, _Halimeda_, _Udotea_ and other members of the Siphoneae. In those fossil stems which possess tubes of two distinct sizes, we cannot as a rule trace any organic connection between the two sets of tubular elements. Transverse septa have been detected in the tubes of some specimens of _N. Logani_. These considerations and the large size and habit of growth of the stem leave one sceptical as to the wisdom of assigning the fossil genus to the Siphoneae. On the other hand, apart from the doubtful sieve-hyphae of Penhallow, the manner of growth of the plant, the concentric rings, marked by a decrease in the diameter of the tubes, the lax arrangement and irregular course of the elements, afford points of agreement with some recent Phaeophyceae. The stem of a _Laminaria_ (fig. 29) or of a _Lessonia_ are the most obvious structures with which to compare _Nematophycus_. The medullary region of a _Laminaria_ or _Fucus_ and of other genera presents a certain resemblance to the tissues of the fossil stems. On the whole we may be content to leave _Nematophycus_ for the present as probably an extinct type of alga, more closely allied to the large members of the Phaeophyceae than to any other recent seaweeds. _Pachytheca._ (A fossil of uncertain affinity.) There is another fossil occasionally associated with _Nematophycus_ and referred by many writers to the Algae, which calls for a brief notice. _Pachytheca_ is too doubtful a genus to justify a detailed treatment in the present work. Although, as I have elsewhere suggested[389], we are hardly in a position to speak with any degree of certainty as to its affinity, it is not improbable that it may eventually be shown to be an alga. Without attempting a full diagnosis of the genus, we may briefly refer to its most striking characters. _Pachytheca_ usually occurs in the form of small spherical bodies, about ·5 cm. in diameter, in Old Red Sandstone or Silurian rocks. In section a single sphere is found to consist of two well marked regions; in the centre, of a number of ramifying and irregularly placed narrow tubes, and in the peripheral or cortical region, of numerous regular and radially disposed simple or forked septate tubes. The tubular elements of the two regions are in organic connection.