SALTER LOWER SILURIAN CRUSTACEA. 93 



Linnean Society of Normandy,' 1861. This shows the entire sur- 

 face of a slab of sandstone, observed by the author of the paper, 

 M. de Brebisson, in a small quarry at Noron, in the Falaise ; which 

 sandstone, overlain by beds of red and brown schist, the author is 

 disposed, with M. Horiere, to consider of Cambrian rather than of 

 Lower Silurian age. The geologic position may be somewhat 

 doubtful ; but at least these sandstones and schists are overlain by 

 the quartzose rocks referred to the age of the Caradoc sandstone, 

 which is now tolerably well known in Normandy. 



The surface of the sandstone is split up by a compound cleavage, 

 and the slabs could not be detached ; casts only and photographs 

 could be taken, but these are quite sufficient to show the whole 

 nature of the imprints. These, though doubtfully referred by M. de 

 Brebisson to gigantic Algce, are (by the consent of four British 

 palaeontologists) clearly referable to the tracks of some Crustacean. 



1 shall endeavour, after describing a similar track from British rocks, 

 to show in what respects the two differ, and in what way the tracks 

 were formed. 



Track of Peltocaris (?) . 



The track to which I would first draw attention was presented to 

 the Museum of Practical Geology * by the late Rev. T. T. Lewis, of 

 Aymestry. He found it in a picturesque glen (Wilmington Dingle), 

 near Chirbury, Salop, in beds of the age of the Llandeilo-flags, which, 

 in various localities near, contain abundance of the characteristic 

 Ogygia Buchii and Lingula granulata. In these beds, up to the pre- 

 sent time, no other Crustaceans than Trilobites have been discovered. 



The surface of the slab, broken unfortunately in two by some 

 meddling collector, was evidently once continuous, and must have 

 been fully 2 feet long by 1 foot broad. On this surface three 

 broad ripple-ridges, with intervening shallow depressions, may be 

 seen ; and the ridges are cut in all directions by the marks to be 

 presently described, while the depressions are left untouched. 



I draw attention to this striking feature, because it at once shows us 

 that the creature swam so near the bottom, that every stroke of the 

 extremity of the tail impinged upon the muddy ridges, and not so close 

 to the bottom but that the instrument, whatever it was, did not strike 

 it when the animal passed over the more depressed surface. 



The tracks vary not a little in length. The deeper ones are often 



2 inches long, and occasionally even more ; the shallower ones some- 

 times not more than 1| inch to less than \ an inch. But all are of 

 the same breadth ; and all, without exception, have been made by the 

 same instrument. They are very numerous, as many as twenty- 

 three being crowded (and crossing each other at all angles) in a space 

 of 4 inches on one ridge. In other portions not above a dozen 

 occupy the same space. They are broad and shallow at the abrupt 

 end, deep and narrower in the middle, and taper a good deal toward the 

 smaller end ; and at both extremities the mud has been driven up 



* Wall-case IV. , T S ' 1 , 

 No. 12. 



