J. W. Dawson—Footprints, ete., on Carboniferous Rocks, 21 
supposed them to have been the work of species of Hymenoca- 
ris. ‘These marks may, however, indicate the existence of some 
free-swimming animals of the Primordial seas as yet unknown 
to us 
Three other suggestions merit consideration in this connection. 
One is that alge and also land plants, drifting with tides or cur- 
rents, often make the most remarkable and fantastic trails A 
marking of this kind was observed by Mr. G. M. Dawson last 
markings of this kind would suffice to give species of Hophy- 
ton, Another is furnished by a fact stated to the author by 
Prof. Morse, namely, that Lingule, when dislodged from their 
burrows, trail themselves over the bottom like worms, by means 
of their cirri. Colonies of these creatures, so abundant in the 
Primordial, may, when obliged to remove, have covered the. 
surfaces of beds of mud with vermicular markings. The third 
is that the Rabdichnite-markings resemble some of the grooves 
in Silurian rocks which have been referred to trails of Gastero- 
pede . for instance, those from the Clinton group, described 
y Hall. 
_ As might be expected, the markings above referred to, when 
im relief, occur on the under sides of the beds, A few instances 
Imitative Markings. 
Rill-marks are often very beautifully developed on the Car- 
boniferous shales and argillaceous sandstones, though not more 
elaborately than on the modern mud-banks of the Bay of 
Fundy,* and they oceur as far back as the oldest Cambrian.t+ 
* Acadian Geology, 2nd ed., p. 26. : 
+ Salter, Journal of Geol. Society, vol. xii, p. 251. 
