202 THALLOPHYTA. [CH. 



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, Halmeda, 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. Trans- 

 verse 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 Lami- 

 naria or Fiicus and of other genera presents a certain resemblance 

 to the tissues of the fossil stems. Oa 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 Nemato- 

 phycus and referred by many writers to the Algae, which calls 



