280 Professor Harkness on 



their margins crenated. A distinct raised line traverses the 

 centre of these tracks, and the interval between this line and 

 the crenations is marked by a succession of other lines at right 

 angles to the centre one ; and these seem to have had their 

 origin in the rings of the body of the annelid. This being the 

 appearance usually presented by the upper side of the flags, the 

 under side of which affords natural casts, the tracks appearing in 

 relief. This perfect state of the tracks does not prevail to the 

 exclusion of more imperfect ones, for these perfect tracks ma- 

 nifest themselves only to a small extent, and can be gradually 

 traced, losing their perfection, until the crenations and the cen- 

 tral line disappear, the track assuming the form of a slightly 

 depressed sinuous line. 



From the nature of these tracks, when most perfect, we have 

 evidence of some features in the structure of the animals from 

 whence they originated. The outer crenated margin resulted 

 from the organs of locomotion of these animals, the cirri, and 

 we may consequently infer that these annalids appertained to 

 the cirrated tribe rather than to the order of Abranchia, as this 

 is represented by the present genus Nais. 



The transverse lines which cover the impressions point out 

 the annelid structure, and at once show that these impressions 

 have not arisen from the wandering of molluscs, the tracks of 

 which are sometimes seen on the surfaces of the sandstone 

 strata; and the central line results from the ventral arch, 

 which in this class runs along the central portion of the lower 

 side. 



The nature of the tracks, as they occur in the lower dark- 

 coloured flags is somewhat different. On the upper surfaces 

 of these they appear also in the form of sinuous furrows, 

 about the same width as the more perfect tracks of the higher 

 flags. Here, however, they rarely present crenations, being 

 regular on their margin, and having, in many instances, the 

 impression of the ventral arch distinct. In these lower dark- 

 coloured flags traces of annelids are not confined to the sur- 

 faces of the strata exclusively. The inner parts of these flags 

 very often furnish traces of annelids to a greater extent than 

 even the surfaces of the beds themselves ; and these too are 

 usually marked by such circumstances as support the infer- 



