17 



communication suggested might be caused by the crawling of Crus- 

 tacea, but which further opportunities and observations have induced 

 him to refer to a different cause, have been noticed only at Wain- 

 lode Cliff on the Severn. There they occur on the uppermost sur- 

 face of the band of micaceous sandstone which represents the " bone- 

 bed," and which appears to have consisted of a fine-grained muddy 

 sand, capable of receiving the most minute impressions, while the 

 pure black clay which forms the superincumbent stratum has pre- 

 served this ancient surface in the most unaltered condition. The 

 ripple-marks produced by currents on the surface of this bed of sand 

 are very interesting, from their perfect preservation, and from often 

 exhibiting two sets of undulations oblique to each other, indicating- 

 two successive directions in the currents, such as would result from 

 a change of tide. 



The impressed markings were evidently produced by living beings, 

 probably by fish or invertebrate animals. To determine their nature 

 Mr. Strickland observed the progression of two species of Littorina 

 among Gasteropodous MoUusca, and of Carcinus M(Bnas among Crus- 

 tacea, but the impressions produced were very different from those 

 under consideration. 



The fossil impressions are of four kinds : — 



1st. Lengthened and nearly straight grooves, about one-tenth of 

 an inch in width, and several inches long, very shallow, with a 

 rounded bottom. These, Mr. Strickland considers as caused by 

 some object striking the surface of the sand with considerable impe- 

 tus. They may often be seen to cut through the ridge of one ripple- 

 mark, and after disappearing in the depressed interval, they are again 

 seen pursuing their former direction across the next ridge. They 

 may have been caused by fish swimming with velocity in a straight 

 direction, and occasionally touching the bottom with the under part 

 of their bodies. 



2nd. Small irregular pits averaging one-fourth of an inch wide 

 and one-eighth of an inch deep. These might have been caused by 

 some small animal probing the mud and turning up the surface in 

 quest of food. Mr. Strickland conjectures that some of the numerous 

 species offish found in the bone-bed may have produced them, the 

 heterocene form of tail common to most of which, Dr. Buckland has 

 suggested, enabled them to assume an inclined position with the 

 mouth close to the ground. 



3rd. Narrow deep grooves, about one-twelfth of an inch in width, 

 the sides forming an angle at the bottom, irregularly curved and 

 often making abrupt turns, apparently formed by a body pushed 

 along by a slow and uncertain movement, such as might arise from 

 the crawling of Mollusks. Mr. Strickland refers them to the loco- 

 motion of Acephalous Mollusca, and supposes that the only shell 

 found in this bed, a small bivalve named by him PuUastra arenicola, 

 might have produced them*. 



* Mr. Strickland describes this species as follows : — " Its form is nearly 

 a perfect oval, depressed, nearly smooth, but with faint concentric striations 



VOL. IV. PART I. c 



