2/6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



P. lineatiis, and P. alternans!' Professor Owen concludes^ that a 

 crustacean like Limulus was nearest to his idea of the kind of animal 

 which left the impressions on the Potsdam sandstone. 



In 1857^ Mr. E. Billings wrote a general paper on the fossils of 

 the Potsdam sandstone, but did not add any observations of 

 importance. 



Dr. J. W. Dawson, in order to test Professor Owen's view that an 

 animal like Limulus may have made the Protichnites tracks experi- 

 mented with a living Limulus at the seashore, causing it to creep 

 about on the sand under various conditions. Summarized, his con- 

 clusions are : * 



1. The conjecture of Owen that they may have been made by a creature 

 somewhat resembling Limulus, is verified by the impressions made by that 

 animal. 



2. The further view of Owen that the grouping of the impressions 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 Professor Owen, that tracks like P. lineatus, 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 some- 

 what differed in different circumstances .... 



3. The animal or animals producing the Protichnites probably 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 com.parative 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 Climactichnites, if the work of a 

 similar animal, were formed under water .... 



5. The suppositions above stated would account for the absence or rarity of 

 remains of the animals which produced the Protichnites .... 



6. If we enquire what animals, known to palaeontologists, have produced the 

 Protichnites, it would seem that no others fulfil the necessary conditions in 

 any particular, except the larger trilobites, for instance those of the genus 

 Paradoxides .... On the whole we may safely conclude that if any of 

 the larger primordial trilobites were provided with walking and swimming 

 feet of the type of those of Limulus, but differing in details of structure, they 

 may have produced both the Protichnites 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 trilobites. In this last case I should suppose 

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



^ Quart. Journ. Geol. Soc. London, Vol. 8, 1852, pp. 214-225. 



* Idem, p. 224. 



' Canadian Nat. and Geol., Vol. i, 1857, pp. 35-39. 



* Idem, Vol. 7, 1862, pp. 276 and 277. 



