Fossil Trades from the Potsdam Sandstone. . 281 



Each separate organ seems usually to have been moved back- 

 ward, and inward, in that way forming the V-shaped ridges. 



The deepness and smoothness of the impressions may be partly 

 the result of similar movements of successive organs, pressing 

 into the same furrow. The longitudinal lines may have been 

 easily made, it would seem, by a rigid candal shield, furnished, in 

 some cases, with bristles, or slender spines. 



The finer traces, nearly transverse to the ridges, may have been 

 produced either by the "recover of the paddles, or by the flowing 

 of the mud," caused by their motion, or the onward movement of 

 the animal. The latter supposition is strengthened, by their ap- 

 pearing only in places, where, from the lowness of the transverse 

 ridges, and apparent washing of material into the depressions, the 

 sand appears to have been of very slight consistency. On this 

 supposition, the long curved track in pi. 1 passes at its sharpest 

 turn, over a firmer spot, but elsewhere the bottom seems to have 

 been much softer. So also in plate 3, 1 below, the track seems 

 have been formed across a series of low ripple- ridges. After all, 

 we must frankly admit, that of thelength, the weight, and the 

 morphological structure of those ancient animals, we learn nothing 

 decisive ; and that with a scientific use of the imagination we get 

 little more than a glimpse of the posterior part of their ventral 

 surface. 



