276 PROF. W. J. SOLLAS ON ICHNITTM [May I9OO, 



refer it to mechanical deformation as the result of pressure was the 

 one which at that time commended itself to my mind. This, 

 however, was not without its difficulties, since the radiate disposition 

 of the wrinkles, which express themselves as Oldhamia, were not 

 likely to have been produced by general earth-pressure ; and had to 

 be accounted for by local action, which was not suggested by the 

 general structure of the rock. Prof. Bonney, perceiving the force 

 of this difficulty, offered several just criticisms of the conclusion to 

 which I had been led. He pointed out that the ridges of Oldhamia 

 are far more definite than the puckerings in an ordinary wrinkled 

 phyllite, and that, while puckered phyllites are numerous, Oldhamia 

 is very rare, and does not occur in ordinary examples of such a 

 rock. Eeference was also made to the fact that the wrinkles of 

 Oldhamia sometimes cross each other, so as to produce a reticulate 

 appearance, which would hardly be the case were they the result of 

 simple puckering. 



Several years later I had the advantage of examining, under the 

 direction of my friend Prof. Watts, the fine collection of Oldhamia 

 made by the officers of the Geological Survey, and preserved in 

 the Museum of Science & Art, Dublin ; among these was one in 

 particular, labelled Oldhamia radiata, which could not possibly be 

 accounted for by rock-folding, and which recalled certain appearances 

 that I had remarked many years previously on the muddy flats of 

 the Bristol Channel, in the neighbourhood of Portishead. Fortu- 

 nately I had made a rough sketch of these at the time, and on 

 referring to this I found that it fully supported the comparison. 

 The recent markings were certainly made by an organism, which I 

 thougbt might be some kind of worm. On requesting my friend 

 Prof. Lloyd Morgan to make a search for the animal, he very 

 kindly consented, and sent me a gathering, not of a worm, but 

 of a small crustacean. Later I had an opportunity of visiting 

 Portishead, and watching this crustacean in great numbers 

 actively at work, so that I was able to trace every step in the 

 formation of its tracks. The little creature lives for the most part 

 completely immersed in the mud, but at intervals it projects its 

 anterior extremity above the surface, and with its long claw snips 

 off a minute strip of the superficial layer, which is doubtless rich in 

 organisms ; it then descends into the mud, and on re-appearing 

 takes another snippet near to, but not quite touching, the side of 

 the first ; this process is repeated till a wheel of gashes round the 

 central area is the result. The excisions are at first quite sharply 

 defined, hut very soon, since they are inclined towards the centre, 

 water begins to slowly drain along them ; this leads to their 

 extension outward in a centrifugal growth, which is accompanied 

 by bifurcation, as tiny tributaries contribute to the main streams. 



Suggestively similar as these markings are to some forms of 

 Oldhamia, it by no means follows that the latter were produced by 

 a crustacean ; were this the case, some other traces of the existence 

 of the organism besides mere superficial markings might be naturally 

 expected to occur in association with Oldhamia, while nothing is 



