THALLOPHYTES, BRYINAE. 49 



seen through an overlying layer. The fissures which bound the separate 

 lumps appear as reticulately connected ridges in half-relief, from which run 

 out numerous small blind processes, the casts of small capillary fissures. 



Still there are a large number of specimens of this class now under 

 consideration, to which Nathorst's explanation can only be applied with 

 violence, or cannot be applied at all. Saporta 1 acted judiciously in entirely 

 giving up Crossochorda, while drawing attention at the same time to the 

 poverty and indistinctness of his opponent's preparations in plaster of Paris, 

 which in fact can in many cases very imperfectly serve the purpose for 

 which they were intended. But if the objects in question are not the 

 tracks of animals, it does not therefore follow, as Saporta maintains, that 

 they are Algae. They may for instance be the excrements of many marine 

 creatures, composed of shaped masses of mud or sand, such as may often 

 be seen in suitable spots on the sea-shore. In this case the object will of 

 course project in half-relief on the upper surface of the beds. The group 

 of Gyrolithes described at length by Saporta 2 may be of this kind. Tubes 

 also of lower animals, if stoutly built and held together by a firm cement, 

 may have something to do with this question. For example, I am unable 

 to accept Nathorst's explanation in the case of Arthrophycus Harlani, 

 Hall, from the Upper Silurians, though it may be a mistake, as Romer 

 also thinks, to regard it as an Alga. That this fossil is no track of an 

 animal is proved at once by the spirally twisted specimen figured by Hall 3 , 

 in which several convolutions lie one above the other and do not intersect 

 one another, as is usually the case. It appears also from the text-that 

 these Arthrophycus-tubes do not project on the under surface of the slab, 

 as Nathorst's view requires, but on the upper. I gather this from the 

 following words of the text 4 : ' since great surfaces are crowded with its 

 fragments ; and these layers are covered only by a deposition of a few 

 inches, when another growth, equally abundant, is found upon the succeed- 

 ing layer.' Schimper 5 also has understood the passage in the same way. 



Such being the state of the case, I am precluded from entering in this 

 place upon a detailed description of the fossil remains belonging to this 

 category, and must limit myself in the following remarks to a brief 

 recapitulation of Schimper's classification and a notice of the chief repre- 

 sentative forms, adding only a few critical remarks on Nathorst's and 

 Saporta's later publications. 



The group of Caulerpiteae contains objects of dissimilar appearance. 

 The Jurassic Phymatoderma which is placed here has just been mentioned. 

 The Keckiae also are referred by Nathorst to tracks of animals. Gyrophyl- 

 lites and Discophorites 6 , fossils from the Lower Chalk of whorled construc- 



de Saporta (1). 2 de Saporta (12). 3 Hall (1), t. 2, f. i c. * Hall (1), p. 5. 



5 Zittel (1). 6 Heer (3). 



