076 MR. W. U. HARDAKER ON A FOSSIL-BEARING HORIZON [DeC. I9I2, 



moiincl of mud or clay scraped backwards with each footstep by the 

 creature in its forward march (see fig. 29, p. 675). 



More rarely, specimens showing the actual impression of the claws 

 are met with. Probably more than one species of crustacean is 

 represented: for, besides the variation in the distance between 

 individual footprints and the variation in the width of the track 

 (such variation being likely because of the different ages and sizes 

 of individuals of the same species), one finds that some differ so 

 much in the relative proportions of the width of the track and 

 the spacings of the markings as to render them almost certainly the 

 work of animals of different species. 



A large number of these tracks have come under my notice. They 

 appear to fall into three groups, which I consider were made by as 

 many different species. In each group are large and small examples, 

 which I interpret as those made by mature and immature indi- 

 viduals. The following seem to be the average measurements of 

 each group : — 



Group A. 



The impressions of the claws in this group appear from careful measure- 

 ments to be opposite one another across the track. The width fi-om tip 

 to tip of the opposed claw-markings is 1'5 to 1'6 cm., and across the inside 

 0'9 to rO cm. The distance between individual footprints in the same file is 

 O'-l cm. — varying from 0'3 to 0"55 cm. 



Group B. 



In this group are tracks made by a larger species of crustacean. The width 

 across the outside of the track is 3 cms., whilst that across the inside of the 

 track is 2 3 cms. The arerage distance between two successive footprints is 

 0-53 ciu. They vary widely, however, from 0-2 to 07 cm. The claw-prints do 

 not appear to pair across the track, and thus nuiy be described as alternigrade. 



Group C. 



In these, the smallest type of crustacean footprint noticed at Hamstead, 

 some of the legs were either shorter or were applied in walking more per- 

 pendicularly to the ground than the others, for a series of markings on the 

 mside of the track is found, in which the claw-prints are shorter and often 

 more bluntly ended than those on each side of them on the outside of the 

 track. The width aci'Oss the outside of the track is I'O cm., the width across 

 the inside 0"5 to 0"7 cm. The distance between successive imprints on the same 

 side varies from O'l to 0"4 cm., but is on an average 0*27 cm. 



On the same surfaces as the last-noticed tracks are often straight 

 line-markings, sometimes parallel to the tracks, but frequently 

 crossing them at a small angle. Although the general direction 

 of these parallel lines approximates to those made by the clawed 

 animal, yet there is no certain indication of their organic origin. 

 Very light bodies moved by water or by the wind along the muddy 

 surfaces would make such lines. 



Other markings may be referred to worms. These worm-markings 

 take the form sometimes of meandering tracks in sandstone, often 

 almost along the planes of bedding, but they may be oblique to it ; 

 at other times they are vertical. In the latter case, heaps of 

 castings are usual on the upper side of the sandstones containing 

 the worm-burrows. 



