THE ELASMOBRANCHS 57 



anchorage in feeding, to stabilize it in swimming, and 

 perhaps to protect it against voracious enemies. These 

 fishes now all possessed powerful jaws with which to 

 seize and devour prey; most of them had a propulsive 

 tail; most of them had improved on the pectoral girdle, to 

 which ultimately were to be attached the muscles that 

 moved the head and pectoral spines or fins; and most 

 of them had developed a large and elaborate brain case, 

 this master nerve ganghon needing room for enlargement 

 as new sensory and motor apparatus was evolved. 



Whether the origin of the ostracoderms is placed in the 

 Cambrian or in the Ordovician period, a long time 

 elapsed before any of their progeny permanently in- 

 vaded the sea— at a minimum, half of the Ordovician and 

 all of the Silurian, or approximately ninety million years. 

 Since both animal and plant food must have been much 

 more abundant in the sea than in the fresh-water lakes 

 and rivers, this long delay suggests that insurmountable 

 physiological limitations rather than competition with 

 the invertebrates held them back from migration. There 

 is nothing about either the Cambrian or Ordovician to 

 afford an alternative explanation. Like the Cambrian, the 

 early Ordovician was characterized by widespread con- 

 tinental submergence, fully half of the present North 

 American Continent being covered by seas in which giant 

 cephalopods— ancestral to the pearly nautilus, cuttlefish, 

 squid, and octopus— and almost equally large trilobites 

 held the dominant position. The Ordovician saw the ap- 

 pearance of the first true corals, starfishes, and clams, 

 and the wide diversification of other invertebrates. Yet 

 the only remains of the ostracoderms are a few famous 

 'first fragments' which have been recovered from Ordo- 

 vician fresh-water deposits near Caiion City and related 

 strata in the Big Horn Mountains and the Black Hills. 

 The period closed with the Taconic revolution, which 

 raised mountains of that name in the area from New- 

 foundland to New Jersey; the highest of these peaks had 



