278 Wisconsin Academy of Sciences, Arts, and Letters. 



side of the track. When tlie Y remains at all regular this is 

 toward the convex margin when the track is curved (vid. plate 1) 



The surfaces of the ripple-like ridges are usually regularly con 

 vex, and this convexity is so nearly equal to the concavity of the 

 furrows between, that from this feature alone, it would be impos- 

 sible to say whether, in a particular case, you were looking upon. 

 the track itself, or upon the cast of it, found on the upper slab. 

 !N"ot infrequently the ridge is narrower than the furrow. Some- 

 times the converse is true. If either slope of the ridge is more 

 inclined than the other, it is usually the posterior one, unless it 

 has been modified by longitudinal lines, when the converse may 

 be true. With this character may be connected the fact that from 

 the way in which the stone breaks in the ridges, there is addi- 

 tional evidence that the ridges in their formation were pressed 

 and moved backward somewhat. 



ITot infrequently the arms of the Y are not formed together; 

 sometimes they alternate for a time^ sometimes an extra arm is 

 intercalated between two Y's, which are distorted, to adapt them- 

 selves to the case. Such cases occur more frequently in curves, 

 but are not confined to them. 



Sometimes the Y-form of the ridge changes to a wavy line, a 

 low W, a regular curve, or a straight line, and that within a short 

 space, in the same track. 



Sometimes the transverse ridges appear to fade out, as if the 

 consistency was insufficient to sustain them after they were 

 formed. 



8. The third element of the tracks are the longitudinal lines. 

 These are seen frequently modifying the tops of the ridges, and 

 forming the most reliable guide, in determining which impressions 

 are the tracks, and which the casts ; also, in which direction the 

 animal moved. The track may also be sometimes distinguished 

 from its cast, by its transverse section being a little concave or 

 depressed. 



These markings may be divided into three kinds. 



First, those quite closely parallel with the sides of the track, as 

 though formed by some appendages dragged over the ridges after 

 they were formed. Of these, some seem to be made by rigid,' 



