276 Dawson on the footprints of lAmidus, 



phalo-thorax in the Limulus is evidently related to this operation 

 of burrowing, and as in many trilobites it seems even better 

 adapted for such a use, it is probable that they also were bur- 

 rowers, which would however suppose the existence of strong feet 

 similar to those of Limulus. 



From the foregoing observations we may, I think, safely de- 

 duce the following inferences respecting Prothichnites : 



(1). The conjecture of Owen that they may have been made by 

 a creature somewhat resembling Limulus, is verified by the im- 

 pressions made by that animal. 



(2). The further view of Owen that the grouping of the im- 

 pressions depended on multifid limbs, and that the number of 

 impressions in a group might indicate specific diversity, is also 

 vindicated by the facts, with this limitation, anticipated by Prof. 

 Owen, that tracks like P. lineatiis, might have been made by any 

 of the animals which made the other impressions, and that if 

 like Limulus they possessed one large pair of feet making the 

 principal marks, and smaller ones occasionally used, the numbers of 

 marks may have somewhat differed in different circumstances. Still 

 it is evident that a species of Limulus having a different number 

 of divisions of the posterior toes, from that to which these remarks 

 relate, might be distinguished by its footprints. 



(3). The animal or animals producing the Protichnites prob- 

 ably resembled Limulus in general form, and in the possession of 

 a strong caudal spine. They probably differed from Limulus in 

 the less breadth or depth of the cephalo-thorax, and in the greater 

 complexity and comparative size of the feet. 



(4). Some at least of the Protichnites were probably produced 

 by animals creeping on wet sand ; but P. lineatus and the Cli- 

 mactichnites, if the work of a similar animal, were formed under 

 water. Tliis accords with the view entertained by Sir W. E. 

 Logan as to the conditions of deposition of the Potsdam sand- 

 stone ; and it is probable that these ancient crustaceans, like the 

 modern Limulus, frequented the sandy beach for the purpose of 

 spawning, and may sometimes have been left dry by the tide. 



(5). The suppositions above stated would account for the ab- 

 sence or rarity of remains of the animals which produced the 

 Protichnites. It is rare to find on the modern beach any frag- 

 ment of an adu'-t Limulus, except on the dry sand above high 

 water mark. The creatures are driven on shore only in storms, 

 and then, owing to the lightness of their crusts, are drifted high 

 on the beach. Their remains are probably to be found in cii:- 



