D.— ZOOLOGY. 59 



hill and valley. But he gives a powerful picture of the tremendous water- 

 erosion which followed this quiet period. Everywhere the top of the 

 Torridonian is eroded ; in one place a channel has been cut 4000 feet deep 

 and miles wide. It is cut down to the Lewisian gneiss underneath, and it 

 seems from Geikie's description that the total depth of erosion was probably 

 10,000 feet. 



I will ask you to consider Zoological evidence that the denuding torrents 

 of this period were succeeded by terribly strong currents in the Cambrian 

 Sea, still powerful in the Ordovician, only returning to tranquillity with 

 the advent of the Devonian. 



In the Silurian we have no signs of the loafing, carp-like fish which so 

 characterise the Devonian. The Silurian fauna, outside some deep- 

 water creatures, with one group of very powerfully swimming fish, consists 

 largely of animals specially adapted to hold on for their lives in a terrible 

 current. Trilobites have centipede claws to clutch with, and a low, flat 

 body shaped like a mud-shoal in a stream, or like Major Segrave's racing- 

 car. The ostracoderm fishes, instead of claws, have to trust to the weight 

 of their massive bones to press them in the mud ; they are armoured to 

 resist the battering of the stones driven by the tide ; they are shaped, 

 like the trilobites, in stream-lines, so that the resistance offered to the 

 current is the least possible, and the resultant pressure holds down flatly 

 and firmly the similar forms of fish, trilobite, motor-car or mud-bank. 

 The fish Cephalaspis is so like the trilobites Paradoxides and Olenellus that 

 Gaskell claimed they proved the descent of vertebrates from crustaceans. 

 The resemblance is the convergence of very diverse animals, shaped to 

 resist destruction from the same 10-knot current. Each is armoured, with 

 a flat stream-line carapace and eyes as far out of the mud as is possible ; 

 in each the parabolic crescent of the carapace is prolonged in two lateral 

 spines, which not only carry back a stream-line revetment around the 

 mobile body, but also are driven splaying into the sand, if the animal is 

 pushed back by the current, and anchor it like the flukes of a grapnel. 

 The four-spined Cambrian trilobite Crepicephalus anchors more securely 

 than those of the Silurian ; it was moored fore and aft, having two earwig- 

 like spines at the end of the abdomen. 4 The armoured fish Pterichthys 

 has a flipper on either side, which, as he is pressed backwards through the 

 mud, will spread out to a click at right angles, like the barbs of a harpoon. 

 If a trilobite be forced from his hold, he coils into a ball and rolls, so as to 

 take glancing blows only. This chance of life, if driven from moorings, 

 seems to be taken by some spherical shells such as the brachiopod Rhyn- 

 conella, and by the extraordinary armoured sponge, Ischadites. The fish- 

 lamprey Palaeospondylus held on by a sucker to a smoothed rock, the 

 crinoids were thickly plated and rooted to the bottom, polyzoa and corals 

 were massive. Before the Caledonian upheaval and the Devonian period, 

 we see a world of mud, clattering stones, and torrential currents. 



I suggest also that the absence of terrestrial life shows that the Torri- 

 donian continents had been smoothed so flat that the Cambrian tides 

 swept over all surfaces except those of recent upheaval. There must have 



4 C. D. Walcott: 'Cambrian Geology and Palaeontology,' iii. Pis. 29-34. 

 Washington, 1916, Smithsonian Institution. Many Cambrian trilobites have posterior 

 epines of various morphology. 



