274 



Dawson on the footprints ofLimulus, 



female of the American Limulus would give a track fully nine 

 inches in width, and much stronger in the principal impressions, 

 while in such a large track the minor impressions of the anterior 

 feet would also be more distinct. 



When the Limulus creeps on quicksand, or on sand just covered 

 with water, so that its body is partly water-borne, it appears 

 principally to use its ordinary walking feet, and the footprints 

 then resolve themselves into a series of longitudinal scratches 

 after the manner of Protichnites lineatus, while the lateral and 

 median grooves retain their distinctness, the former being appa- 

 rently even of greater proportionate depth than on firm sand. 

 This kind of impression is represented in Fig. 2. 



I^ffff.^ 



« ^11^^^ 



■ 



Fig. 2. 



Fig. 3. 



When placed in shallow water, just covering the body, the 

 creature used its flat abdominal swimming feet, and though the 

 impression made was very faint, and not readily observed under 

 water, it was obviously very different from those before men- 

 tioned, agreeing with them only in the lateral and median 

 grooves, while between these were series of furrows extending ob- 

 liquely from each side of the middle groove, and resembling ripple 

 marks. (Fig. 8.) These were produced by the sand swept up by the 

 swimming feet. The appearance was not unlike that of the im- 

 pressions found by Dr. Wilson in beds containing protichnites, 

 and recently described by Sir W. E. Logan in this Journal, under 

 the name Climactichnites, except that in the track of Limulus 

 the lateral and medial lines are furrows instead of ridges. The 

 supposition in respect to CUmacticJmites has been, that it is 

 possibly the track of a gasteropod, or of an annelide ; but my re- 

 cent observations of Limulus show that it mai/ have been pro- 

 duced by a crustacean moving by broad swimming feet, having a 

 median notch like that in the largest pair in Limulus. To pro- 



