270 EEPORTS ON THE STATE OF SCIENCE. 



diflBculties. We have to deal with natural casts giving the outward form 

 only and no trace of internal structure, and secondly we find that purely 

 physical causes give rise to forms closely resembling those attributable 

 to organisms. 



It is evident that a study of the tracks of recent animals is a most 

 essential preliminary. Here the work of Professors Nathorst and Hughes 

 will be of great assistance ; both these authors give descriptions and 

 carefully drawn and photographed figures of actual tracks of known 

 animals.' 



It will be found, however, that the record of a few score of species 

 will cGv'er but a small part of the variety of tracks, and it will readily 

 be seen that the varying circumstances under which the tracks were 

 produced must greatly modify their form. 



In the meantime a record of some of the fossil tracks and other 

 markings found in the Trias, more or less systematically arranged, may 

 be of use. 



The forms that are readily identified may be divided into tracks of 

 animals and casts of plants, and markings evidently inorganic which 

 may be classed together as such and subdivided according to their 

 resemblance to forms which we may now observe in course of production, 

 such as stream courses, rain pittings, ripples, &c. 



But between these principal groups we shall have to deal with a 

 number of forms of very doubtful origin, simulating orgmic forms or 

 markings possibly attributable to organisms, also with a number of 

 forms whose origin is very uncertain, but which do not simulate any 

 known organic forms. We shall then have 



Organic. — Tracks of invertebrates. 



„ Casts of plants. 



0/ Doubtful Origin. — Markings simulating plant forms. 

 „ „ „ „ animal „ 



„ ,, Parallel or concentric lines, fillets or -ridges. 



,, ,, Lines, &c., neither parallel nor concentric. 



Inorganic. — Rain pittings, and similar forms. 



„ Ripple marks. 



„ Wrinkles and other irregular markings possibly 



due to movements in the rock itself. 

 „ Pseudomorphs. 



Markings of Organic Origin. 



Tracks of Invertebrates. — The most common of these are sinuous 

 markings in relief about 2 mm. or more in width, roughly semicircular 

 in each section, with a slight groove on each side (indicating a ridge in the 

 original track). The windings are frequent and intricate. They are often 

 referred to as ' annelide tracks,' but the tracks of recent worms are com- 

 paratively free from short turns, and these bear more resemblance to the 

 tracks of small Gasteropods, so frequent between tide-marks on a muddy 



' ' On some Tracks of Invertebrates, &c., and their Palaeontological Bearing,' 

 by A. G. Nathorst, Kon. Seenzka Vet. Aka,d. Handlinffor, Band 18, No. 7, 

 December 30, 1880. 



' On some Tracks of Terrestrial and Freshwater Animals by Professor T. McK. 

 Hughes, Quart. Tourn. Geol. Soc, vol. xl. p. 178. 



