334 PROCiEEDrN'GS OF THE GEOLOGICAL SOCIETY. [JuilC 19, 



of the Silurian epoch was, in Britain, marked by the silting up of 

 shallow seas, to form land-locked estuaries for the deposit of the Old 

 Red Sandstone. 



Even the characteristic tide-wash-mark (if I may coin a triple 

 term) is present ; and fig. 1 shows as good an instance of this shore- 

 character as can be seen on the paving-flags of Forfarshire, or, still 

 nearer, on those of the Yorkshire Coal-measures. 



And on these surfaces Mr. R. Banks, of Ridgbourne, found (I am 

 ashamed to say how many years back) the beautiful impressions on 

 the two slabs upon the table, for which I am much indebted to him. 

 To my mind these slabs show all but conclusive evidence of the 

 tracks of Ludlow fishes, two (if not three) species of which have been 

 described by Prof. Huxley and myself in a former volume of the 

 Journal*, from this locality. My reasons for believing the tracks to 

 be the work of Pteraspid fish f are partly drawn from the character 

 of the imprints themselves, and partly from the fact that, except the 

 large Pterygotus, which occurs in the same bed, no creature of size 

 or weight enough to make these imprints is known, either from King- 

 ton or elsewhere in the Downton Sandstone. Of course the argu- 

 ment from negative evidence will be stronger with some than with 

 others. But if (for the purpose of inquiry) it be admitted that our 

 choice is narrowed to the kinds of animals known to have existed at 

 the period, I should have no hesitation whatever in rejecting the 

 crustaceans and moilusks, and deciding for the fish ; and Pteraspis and 

 its allies are the only fishes known to exist in Upper Ludlow Rocks, 

 except the small Dogfishes or Sharks (if indeed they were so, for 

 we do not know their teeth, and they may have been Acanthodians) 

 which are found in the Ludlow bone-bed. At Kington, though I did 

 find the bone-bed by its peculiar shells, I cannot remember that it 

 showed any traces of fish. But fin-spines of fishes do occur in 

 plenty in these flaggy sandstones; and the reasonable inference would 

 be that they may have belonged to Pteraspid fish, unless it can be 

 shown that such fishes could not have had defensive spines to their 

 fins. 



I wiU describe the slabs first, of which I have only two : — ^Fig. 1 

 represents the smaller, about 18 inches square by -J- of an inch thick, 

 and shows traces of three tracks made by a larger individual than 

 those which have left traces on No. 2 (see fig. 2). The latter is a 

 much thicker slab, with many tracks (confused for the most part) 

 over a space a foot square. It has two contiguous layers imprinted by 

 them (a and (3), separated by only a thin layer of sandstone ; while 

 a third or upper layer (y) is free from markings, so far as it is seen. 

 The tracks on this larger slab can, I think, best be interpreted by 

 reference to the smaller and more perfect one. But with this help 

 they enable us to observe some characters not plainly visible on No. 1. 

 Pig. 1 shows three uiidulating lines of imprints, neither parallel 



* Quart. Journ. Greol. Soc. vol. xii. p. 100. 



t A fine slab of markings, probably due to Cephalaspjs, exists, or did exist a 

 few years back, in the great Jerrayn Street Collection. It is from the Cornstone 

 beds of the OJd Red. A still finer one is in the Worcester Museum, 



