338 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 19, 



instruments were driven first inwards bo as nearly to meet, and then 

 opened suddenly outwards with various degrees of force. The stroke 

 was, if I interpret the direction of the tracks rightly, a little back- 

 ward as well as inward in each case, and the return stroke also 

 backward. 



The evidence, then, as far as I understand it, is, that one pair of 

 organs placed near each other, and acting in a direction transverse 

 to the progress of the animal, were the means by which the creature 

 was propelled through the shallow water. Also that a posterior lobe 

 or supplementary organ, shorter than the other, touched ground in 

 the deeper imprints ; but as it occurs only behind the deeper ones, 

 which happen in this case to be on the left side, the inference is that 

 there were a pair of such shorter organs immediately behind the 

 longer pair. 



Such an arrangement excludes, of course, the gigantic crustaceans 

 described by Prof. Huxley and myself*, and now being so thoroughly 

 examined by Mr. Henry Woodward. The Pterygoti are indeed plen- 

 tifully found in the Kington Sandstones. Perhaps the largest of 

 them all, at least the P. gigas, rivals the great Forfarshire species 

 in size, and must certainly be 6 or 7 feet in length, as I at first sup- 

 posed. But the struggles of such an animal in the shallows would 

 leave, not a bilobed imprint, at times unequal-sided, but a number 

 of impressions, arranged along a central line. Such are shown in 

 Prof. Owen's illustration of the Protichnites of Canada, which that 

 distinguished naturalist refers without much hesitation to large 

 Limuloid or Eurypteroid Crustacea. In that view I thoroughly 

 agree with him, and believe we have yet to discover the ancestor of 

 all the Pterygoti in the Potsdam Sandstone. 



That we may eliminate all the contemporary crustaceans from con- 

 sideration, I may mention that I have myself described tracks of the 

 Phyllopod Crustacea found in Lingula-flag and Silurian rocks, and 

 made with double or triple forked caudal extremities (Quart. Journ. 

 Geol. Soc. vol. X. p. 211, vol. xviii. p. 347). Such impressions have 

 nothing in common with these. They were made in the opposite 

 direction to that of these imprints, namely, fore and aft, in the line of 

 the track itself. 



But if we turn to the abnormal Ludlow fishes, Pterasj)is (the 

 earliest known of these) is common enough in the Kington Sand- 

 stones, as Mr. Banks's researches first proved to usf. These seem 

 to give a clue to the impressions before us. If such fish, evi- 

 dently fond of shallows, were endowed with stiff defences to their 

 pectoral or ventral fins, they might produce with them somewhat 

 such a pair of combined strokes as would lift the animal forward or 

 backward. The slight obliquitj^ of the stroke is not against the 

 supposition ; for the motion of a fish's fin is not quite in a direct line, 

 but more like that of a screw propeller. 



That Pterasjyis had such stiff defences I do not know. But a not 

 very distant ally among these abnormal cuirassed fishes (PtericTitJiys) 



* Monograph of Pterygotus and Eurypierus, Mem. GeoL Survey, 1859. 

 t Quart. Journ. Geol. Soc. vol, xii. p. 93. 



