ANCESTRY OF THE PEARLY NAUTILUS. 189 



coiled chambered shells which almost any person would iden- 

 tify with Nautilus. They really have all the essential char- 

 acters of Nautilus; but you will notice that they are not 

 closely coiled ; we do not find each whorl overlapping and 

 concealing all the others ; and the last whorl is even a little 

 separated from the preceding one (Lit-u-i'-tes). Many others 

 are coiled, but somewhat loosely, and the siphuncle is one side 

 of the center sometimes close to the outer margin (Gy-roc'-e- 

 ras). Still others are curved enough to form one whorl, but 

 not properly coiled, and the siphuncle is close to the outer 

 margin (Cyr-toe'-e-ras). But we have not time to trace all 

 the varieties of the type of chambered shells even among 

 the Silurian limestones. We may have future opportunity to 

 glance at the history of this type, and show the great improve- 

 ments made in complication and decoration, during the Meso- 

 zoic ages. 



In strolling through the quarries excavated in the Niagara 

 limestone in the suburbs of Chicago, for instance, or at 

 Joliet or Waukesha our attention is constantly arrested by 

 the remains of shells, corals, and criuoids. The bivalve shells 

 are chiefly Brack' -i-o-pods. They are lower in rank than clams 

 and river mussels. They may always be known" by having 

 the beak and hinge in the center of the valve, with the 

 valves presenting the same slope and curvature each way 

 from the beak. They may also be known by having one 

 valve more swollen than the other. Many also, have a deep 

 depression (Sinus) along one valve from the beak to the 

 opposite margin, and a corresponding elevation (Fold) in the 

 opposite valve. Brachiopods are now nearly extinct. The 

 univalve shells are mostly Gas'-ter-o-pods. These are higher in 

 rank than clams and mussels."" 



The Criuoids were plant-like forms (Zo'-o-phytes) but strictly 

 animalsTn~natufe^ The most common kinds were footed to 

 the muddy sea-bottom, as is proved by specimens quite abund- 

 ant in Niagara shales at Waldron, Indiana. The old roots 

 are found going down into the clay like the roots of an oak. 

 Above the root rises a stony stem, ten or fifteen inches high, 



