Dawson on the foot'prints ofLimulus. 211 



cumstances favourable to their preservation, only on the muddy 

 bottoms at a distance from the sandy shore. Young individuals 

 only appear to frequent the sand in summer, and occasionally to 

 be imbedded in it. 



(6). If we enquire what animals, known to palaeontologists, have 

 produced the Proticbnites, it would seem that no others fulfil the 

 necessary conditions in any particular, except the larger trilobites, 

 for instance those of the genus Paradoxides. It is true that we 

 know nothing as yet of the feet of these creatures, but it seems al- 

 most certain from analogy that they must have possessed such 

 organs. Nor have these trilobites a caudal spine like that of 

 Limulus ; but here again Mr. Billings points out to me that the 

 pygidium of Paradoxides is narrow and spine-like, though I should 

 think not sufiiciently so to form the very distinct median groove of 

 Proticbnites, unless indeed the creature was in the habit of walking 

 with this organ pointed downward. On the whole we may safely 

 conclude that if any of the larger primordial trilobites were pro- 

 vided with walking and swimming feet of the type of those of 

 Limulus, but differing in details of structure, they may have pro- 

 duced both the Proticbnites and the Climactichnites. On the 

 other hand, it is quite possible that these impressions have been 

 formed by crustaceans yet undiscovered, and approaching in some 

 respects more nearly to Limulus than any of the known trilo- 

 bites. In this last case I should suppose that the animal in 

 question had a flatter or more shallow cephalo-thorax than that 

 of Limulus, proportionately stronger and perhaps more divided 

 feet, and a stouter caudal spine. 



It is scarcely necessary to observe that the footprints ofLimulus 

 differ materially from those of the higher crustaceans, and also 

 from the galleries formed by many small burrowing Crustacea. 

 With these last Mr. T. Rupert Jones, in an interesting article in 

 the "Geologist" for April, seems disposed to compare Climac- 

 tichnites Wilsonii ; but this appears to me to have more the 

 character of a surface impression, though what appear to be 

 galleries of small crustaceans are also found in the Potsdimi sand- 

 stane. The "iVereiVes" of Emmons,* from the Taconic rocks of 

 that author, also resemble in some respects the sub-aquatic trails 

 of Limulus, and may be tlie work of Trilobites ; and the same re- 

 mark applies to some of the markings from the Clinton of New 

 York, figured by Hall,f and referred to Crustacea and worms. 



•Agriculture of New York, Vol. I. 



t Palaeontology of New York, Vol. II, PI. 13 to 16. 



