34 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 'jj 



the only one known which bears any great resemblance to those here 

 described. 



A second slab (No. 11,141, U. S. N. M.) which was originally a 

 part of that bearing the type, has on its surface a continuation of 

 the Palcohelcura trail evidently made by the same individual. It 

 differs from the type in having only a single toe mark on each side of 

 the tail drag for one-half of its linear extent, the remaining half show- 

 ing two imprints. In only three or four instances are all three toes 

 registered. This serves to emphasize the necessity of securing abun- 



FiG. 22. — Mcsichnium bcnjamini. Type, No. 11,155, U. S. N. M. 

 Diagram of trackway, i natural size. 



dant material for the study of fossil tracks if the chances of error 

 are to be reduced to the minimum. 



Genus MESICHNIUM, new genus 



Generic characters. — Digital formula unknown. Row of regularly 

 spaced oval depressions between the parallel lines of tracks. 



MESICHNIUM BENJAMINI, new species 



Plate 12, fig. 2 



Type. — Catalogue nmnber 11,155, U. S. N. M. Consists of a 

 small slab on which is a trail about 300 millimeters in length. 



