REPORT OF THE STATE PALEONTOLOGIST 1902 905 



Comparison with the Chitons. Professor Walter Faxon sug- 

 gested to me, on seeing the drawing, figure 1. a comparison of 

 the oval impressions with the outline of the ventral surface 

 of the shell of the modern larger Chitons. Some of the Chitons 

 are somewhat narrower anteriorly than posteriorly; but the 

 oval grooved impression which their ventral surface would 

 give has much resemblance to that of the sedentary impressioD 

 of Climactichnites. The mantle of the Chitons extends beyond 

 the shells; they have a powerful foot for attachment to rock 

 surfaces; though habitually attached to rock surfaces, they can 

 crawl very slowly over sand; they can roll themselves up into 

 a ball, as did many trilobites; the retractile forward hitching 

 movements of the foot of a large Chiton, moving inch by inch, 

 might have given rise to the smoothened, cross-ridged trails 

 known as Climactichnites; and the sedentary animal with re- 

 tracted foot, the terminal impressions seen at Mooers. Xo other 

 existing form seems so readily capable of producing the assem- 

 blage of phenomena. But it is equally possible that some large 

 sea slug of the order of Aplacophora may have produced the 

 trails and the terminal impression, though the modern repre- 

 sentatives of these primitive gastropods are not littoral ani- 

 mals. Further than the suggestion of a molluscan origin for 

 these terminal impressions and the connected trails, it does not 

 seem possible at present to seek definite conclusions. It is 

 possible that a more detailed study of the terminal impressions 

 at Mooers with proper illumination will bring out faint traces 

 of impressions not here described, and that from these diagnos- 

 tic characters may be obtained. 



The sedentary impressions terminate the history recorded by 

 the trails. What became of the animal after making the seden- 

 tary impression is a matter of conjecture. A jellyfish or a naked 

 mollusk such as Doris, becoming stranded, may wither away, 

 leaving no recognizable vestige of its substance; this would 

 hardly be the case with the Polyplacophora. It may be sup- 

 posed that the animals which made the sedentary impressions 

 on the Potsdam sandstones at Mooers floated away with the 

 incoming tide. On the other hand, it is quite unlikely that in 

 the porous sandstones of this area — for porous they must have 



