20 PROF. W. C. WILLIAMSON ON SOME UNDESCRIBED 



at that time to discover any sucli in situ, at a later date 

 Mr. Kavannah was more successful. He then obtained 

 some fine examples from the under surfaces of some of 

 these undisturbed beds^ making it certain that the objects 

 immediately to be described belong to the Yoredale 

 division of the Carboniferous strata. 



Like so many allied remains obtained from Silurian 

 deposits^ these objects standout in bold relief from the 

 inferior surfaces of the rock-layers^ of which their sub- 

 stance is a mere extension. The peculiar sculpturings 

 characterizing these convex surfaces are wholly superficial^ 

 indicating that they are but casts of concave tracks once 

 existing on the surface of the subjacent stratum. The 

 dimensions of those excavated tracks are faithfully, though 

 invertedly, represented by the prominent configurations of 

 the objects before us. 



The specimen (Plate I. fig. i) represents a slab twice the 

 size of the photograph, upon which are three more or less 

 defined meandering ridges. The longest of these runs from 

 « to a. A considerable portion of it is almost obliterated ; 

 but at each extremity it preserves its characteristic features. 

 At b and c are two shorter ones, each of which commences 

 in an undefined irregular elevation ; b near the centre of 

 the slab, and c at d ; but both acquire their peculiar 

 sculpturings at the extremities b and c. Assuming that 

 the creatures which made these tracks moved towards the 

 lower margin of the specimen, the appearances suggest 

 that in the cases b and c they terminated their strolls by 

 sinking into the sand, as many recent invertebrates do, 

 on reaching the spots where each of two of the tracks end 

 in an irregular mass, as represented at d. 



The average diameter of each of these tracks is from 

 five to six tenths of an inch. Their elevation, represent- 

 ing the depth of the original tracks, is sometimes four 



