366 THE CANADIAN NATURALIST. [Oct. 



and in sand a quantity of this material is thrown out behind, 

 which in a cast would have left two hollows, not present in the 

 fossils ; but should a Limulus burrow in fine mud, which would 

 become diffused or washed away as thrown out, then the appear- 

 ance would be not unlike that of these fossils. The front of the 

 carapace would give the rounded, anterior end ; the two rows of 

 walking and swimming feet would form the depressions with trans- 

 verse stride; and the only addition would be the mark of the 

 caudal spine of Limulus, of which there is no trace in the fossils. 

 The animal required would therefore be a crustacean, having feet 

 and habits of life generally resembling those of Limulus, but 

 without a caudal spine. The only known animals of the period 

 that could have fulfilled these conditions are the Trilobites; and 

 since the interesting discovery, by Mr. Billings, of the feet, or bases 

 of the feet, of Asaphus, the objection to this view which might 

 have been taken from our ignorance of the feet of these animals, 

 no longer exists. The feet of Asaj)Jius, in short, appear to con- 

 stitute just such a double series of laminae as would necessarily 

 produce markings like those referred to. 



From the great depth of these burrows, and the indications of 

 shallow water in the vicinity of a shore presented by the shrink- 

 age-cracks, I would further consider it probable that these holes 

 were places of incubation ; and that the Trilobites carried their 

 spawn attached to their swimming-feet, and were in the habit of 

 resorting to shallow water for the purpose of incubation. 



The above remarks apply more especially to H. Grenvillensis. 

 I can speak with less confidence of Professor Hall's species ; but 

 the only specimen which I possess of the R. bilohatus of New 

 York, differs from the Grrenville specimens only in the proportions 

 of length and breadth ; as might be expected, if, as is probably the 

 case, it is the track of a different species. My bilobate impres- 

 sions from Nova Scotia have been produced by a small animal ; 

 perhaps the little species of PhilUpsia which occurs in the same 

 formation. Mr. Salter's Arenicola from the Longmynd wants 

 the transverse markings, and the impressions are somewhat separate, 

 so that they may be of a different character from the others, I think 

 it quite likely, however, that the more elongated species of Ruso- 

 phycus, in the Clinton of New York, may be casts of tracks of 

 Trilobites, and I have long believed that a similar explanation 

 will apply to some at least of the supposed fucoids known as 

 Artlirophycus. 



