Fossils of the Potsdam Sandstone, ol 



sand, and shallower while partly raised by the legs in each move forward. 

 Accordingly, Mr. Logan states, " a feature common to all the grooves ii?. 

 that each repetition or homologue of the foot-prints is accompanied with 

 a deepening and shallowing of the grooves, giving it the appearance of 

 a chain of shalloio troughs, which, when the impression is light, are 

 separated from one another by intervals of the ungrooved surface." 

 The foot-prints of all the tracks are small and sharp, as if made with a 

 pointed instrument, like the hard sharp extremities of a crab's claw, and 

 instead of seven legs upon each side the animal may have had only two, 

 three, or four, wath two or three points at the end of each. Whether this 

 was so or not cannot be yet determined. 



In another kind of these tracks the groups of impressions are not 

 opposite, but appear as if the animal had moved the legs upon one side, and 

 then those of the other alternately, throwing itself forward a little each 

 time, with a waddling motion, and making with each move, a plunge in the 

 sand, Professor Owen has given to these last mentioned tracks the name 

 of Frotichnites alternans. In another species there are eight prints 

 instead of seven. Another shews three grooves, as if the animal had partly 

 floated in the water, dragging its legs by its side. In one, where there is a 

 bend in the track, the median groove verges to the outside of the turn and 

 partly obliterates some of the foot-prints. This track appears to shew that 

 the median groove was made by the tail rather than the body of the animal. 

 In Professor Owen's paper above cited, he has classified these tracks into 

 six species, as follows : 



1. Protichnites septem-nofatus, (seven marked.) 



2. Protichnites octo-notatus, (eight marked.) 



3. Proiichnites latus, (broad.) 



4. Protichniies multinotatus, (many marked.) 



5. Protichnites lineatus, (linear.) 



6. Protichnites alternans, (alternate.) 



In discussing the probable nature of the animal by which these tracks 

 were made, he states in substance that three replies or suppositions may be 

 given. 1st, Either each print was made by the extremity of a single limb, 

 which would give either seven or eight pairs of legs to the animal, according 

 to the species ; or, 2ndly, certain pairs of the limbs were bifurcate, as in 

 some insects and crustaceans, another pair or pairs being trifurcate at their 

 extremities ; and each group of impressions was made by a single so-subdi- 

 vided limb, in w^hich case we have evidence of a remarkably broad and short 

 hexapod or six legged creature ; or, 3rdly, three pairs of limbs were bifur- 

 cate, and the supplementary pits were made by small superadded limbs, as 

 in some crustaceans ; or, 4thly, a single broad fin-like member, divided at 

 its border into seven or eight obtuse points, so arranged as to leave the 

 definite pattern described, must have made the series of those groups, by 

 successive applications to the sand. He thinks the latter hypothesis the 

 least probable of all, and with respect to the first, says, '' I confess to much 

 diificulty in conceiving how seven or eight pairs of jointed limbs could bo 



