1864.] DAWSON ON THE GENUS RUSOPHYCUS. 365 



To this description it is only necessary to add, that, in comparing 

 a large number of specimens, many diversities are apparent in the 

 relief of the forms, in the extent of the longitudinal furrow, and in 

 the number of the transverse wrinkles. The two lobes are also 

 most frequently slightly unequal in their relief ; and some of the 

 specimens slope gradually at one end, and are thus somewhat elon- 

 gated. In all cases, however, the general form is the same, the 

 longitudinal and transverse furrows are constant, and the former is 

 always more strongly marked at one extremity of the fossil. The 

 specimens have no indication of a stem or stalk ; though a cast of 

 a worm-burrow or shrinkage-crack sometimes simulates such an 

 organ. 



In viewing these fossils and the surfaces of the beds containing 

 them, it appeared evident that they are in reality casts of hollows 

 or holes excavated in clay, and filled with sand which has taken 

 and retained in its consolidated state the impression of their forms. 

 The supposed fossils project from the lower surface of the sand- 

 stone, where this rests on friable, dark grey shale. They have the 

 same appearance with the surfaces of the beds of sandstone, and 

 show no traces of organic matter. There are on the same surfaces 

 casts of worm-tracks, also in relief, and which sometimes extend 

 over the specimens of Rusophi/cus. There are also on these surfaces 

 rows of wrinkles, or casts of furrows similar to those of Rusophycus ; 

 and some of these form trails to or from the ends of the latter. 

 (Fig. 2, a.) Casts of shrinkage-cracks in relief, also occur on the 

 same surfaces. Large specimens of Rusophycus sometimes overlap 

 small ones in such a manner as to show that they must have been 

 scooped out of the clay. On the other hand, if the supposed 

 fucoids were really of that character, they must have been solid 

 masses or vesicles, and in the former case must have left some 

 trace of organic matter, while in the latter they could scarcely have 

 impressed themselves so deeply on the clay. 



These appearances can, I think, be explained on the suppo- 

 sition that some animal crawling on the soft mud at the bottom 

 of shallow water, by means of feet which made a double series of 

 transverse marks, was in the habit of excavating deep burrows for 

 shelter or repose, and that these burrows were filled with drifted 

 sand constituting the lower part of what is now a thin bed of dark- 

 colored sandstone. The burrowing of the modern Limulus, as 

 described by the writer in vol. vii of this journal, would produce 

 a similar effect. I have not seen the burrows of Limulus in clay ; 



