\ 
found in the Potsdam Formation. 21 
tracks, but it left the mould of them on the underside of the 
upper bed, and by plaster one from this we have obtained the 
true form of the original trac 
These tracks consist of a Sema of parallel ridges and fur- 
rows something like ripple marks, which are arranged between 
two narrow continuous paralle ridges, giving to the whole im- 
pression a form very like that of 
a ladder, and as the whole form 
is usually gently sinuous it looks 
ike a ladder of rope. e sur- 
face obtained shews six different 
— (fig. 1,) the longest of which 
about thirteen feet, but the 
are all of the same breadth, and 
they may all have been: im- 
pressed by one and thesame ani- 
mal. The breadth of the trails & 
is about six inches and three- = 
quarters to the outer sides of 
them. 
The transverse ridges and fur- 
3. 
3, 4, 5.) When straight and reg- 
ular they measure about an inch 
and three-quarters from the mid- 
le of one furrow to that of the % 
next. The height of the ridge is — 
of the Nevins is about ah inch 
tops of the ridges, and the bot- 
toms of the fatrowa are somewhat 
rounded, 
Though the transverse rid a 
are occasi eeeaty straight, (fig. 2 
chord of the curve i m quite at pit ang es tot 
tion of the aaiel se oa one end of the poe in the 
greatest ae ol much as two inches and a 
