| Nematophycus Logani (Daws.) | Lower Devonian of Gaspé. | |
| Silurian [Wenlock] of England. | ||
| Silurian of New Brunswick. | ||
| N. Hicksi (Eth.) | Silurian. (Wenlock) of N. Wales. | |
| N. crassus (Daws.)[388] | Middle Devonian of Gaspé and New York. | |
| N. laxus (Daws.) | Lower Devonian of Gaspé. | |
| N. tenuis (Daws.) | Lower Devonian of Gaspé. | |
| N. Storriei (Barb.) | Silurian (Wenlock) of Wales (Cardiff). | |
| N. dechenianus (Pied.) | Upper Devonian of Germany (Gräfrath). | |
| N. Ortoni (Pen.) | Upper Erian of Ohio. |
In summing up our information as to the structure of Nematophycus we find there are certain points not definitely settled, and which are of considerable importance. The few recorded instances of spore-like bodies by Penhallow and Barber are not satisfactory; we are still ignorant of the nature of the reproductive organs. Such instances of lateral appendages as have been referred to do not throw much light on the habit of the plant. So far as we know at present the stem of Nematophycus was not differentiated internally into a cortical and central region. It may be that the specimens have been only partially preserved, and the coaly layer which occasionally surrounds a stem may represent a carbonised cortex which has never been petrified. The large and loosely arranged tubes constitute the chief characteristic feature of the genus; in some cases (N. Logani) there is an accompanying plexus of smaller hyphae, in others (N. Storriei) there is no definite division of the tissue into two sets of tubes of uniform size, and in N. Ortoni the tubular elements are all of the large type.
Penhallow has recognised the branching of large tubes in N. Logani and N. crassus giving rise to the small hyphal elements. In most specimens, however, no such mode of origin of the smaller tubes can be detected. The spaces which interrupt the homogeneity of the tissues in some forms have been described as branching depots, on account of the frequent occurrence in these areas of much branched hyphae. The function of these spaces (fig. 39, D, and fig. 40) may be connected with aeration of the stem-tissues.
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.
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.