THE KING CRAB'S GRANDFATHER. 195 



formed. We may feel confident that if any shells or corals 

 had been originally inclosed in the sediments, they would 

 have been destroyed. Especially would carbonate of lime 

 have disappeared. Therefore, we are not certain that no cham- 

 bered shells existed before the Cambrian. They may have 

 existed. They may have been so formed and constituted as 

 to show that the Cambrian species were not suddenly intro- 

 duced, but made their appearance in such graduated succes- 

 sion as evolution implies. Here, at least, is a possibility 

 which prevents us from feeling confident that the Cambrian 

 Orthocer'atites were introduced by a sudden creation. 



In these lowest Cambrian strata are, also, still other forms. 

 Here we find Gas'-ter-o-pods univalve shells coiled up. These, 

 too, are well advanced from any humble beginning of Gas- 

 teropods in case they began in a humble way. The same 

 queries arise as in the case of chambered shells. Now, to 

 recapitulate, we find in these lowest, fossil-bearing strata, re- 

 mains of several types of animals appearing to our knowledge 

 for the first time, but all well advanced beyond the lowest 

 grades of the orders to which they belong. Here, in the very , 

 lowest strata, are Trilobites ; Lingula and some related genera / 

 of Brachiopocls, as well as Or' -this, quite a different genus, and !{ 

 perhaps Worms. In the Potsdam Sandstone are, also, Trilo- 

 bites, as well as other crustaceans, Grap'-to-lites (branching plant- 

 like animals with a horny skeleton), Sponges (of calcareous 

 kind), Lingula and other Brachiopods ; Pteropods; Gastero- 

 pods and Orthoceras. I enumerate them simply to make clear 

 the fact that at the very dawn of the Cambrian Age numer- 

 ous types well advanced in rank, suddenly appeared. You 

 will notice, however, that several important types of animals 

 were absent. Here were no corals, no crinoids, no Bryozoans, 

 no Lamellibranchs. 



So far we have confined our attention to the lowest group 

 of the Cambrian rocks, composed of the Acadian or St. John 

 formation and the Potsdam Sandstones. Next above the Pots- 

 dam is the Calciferous formation. It is very conspicuous 

 along the bluffs of the Upper Mississippi, where it forms gen- 



