ANCESTRY OF THE PEARLY NAUTILUS. 187 



above it proclaiming intelligibly that the waters were dis- 

 turbed when the Conglomerate was deposited, more quiet 

 when the materials of the sandstone were laid down, and still 

 quieter when the fine sediments settled down which formed 

 the Clinton marls and the Niagara shale. Please bear, in mind 

 this law of the succession of different kinds of sediments. 



Now let us examine the contents of these Silurian strata. 

 The geologist has been around with his hammer, and looked 

 them through and through. True, he has not broken to 

 pieces one-millionth of the Silurian rocks of the country ; but 

 he has broken an immense number of samples from many 

 localities and many horizons ; and we justly believe that the 

 system has been fairly probed. In the lowest beds the 

 Oneida Conglomerate nothing of much importance has been 

 found. This does not surprise us, for shells and corals must 

 have been ground to powder, had they been mingled with the 

 rolling stones of which that formation is composed. The 

 Medina sandstone was fine enough. to allow the accumulation 

 of some organic remains. We find small heaps of petrified 

 sea- weeds. One sort is regularly jointed, and presents a some- 

 what elegant appearance (Ar-throph'-y-cus). We are much 

 interested to be able to discover which way the currents set 

 over the soft sand. In New York it is common to find a 

 sandstone surface with a little shell lying, convex side up, 

 and beyond it a train or drift of sand a few inches long, and 

 diminishing to a point. How similar were the conditions 

 of the sandy beach then and now! How surprising that a 

 little ridge of soft sand formed millions of years ago, should 

 have been so carefully preserved through all the storms and 

 revolutions of the world to our day ! 



It is in the limestones, and especially the Niagara Lime- 

 stone, that we find the relics of the ancestors of the Pearly 

 Nautilus. It may seem strange that most of them are straight 

 rather than coiled. But their structures are the same, and 

 the coiling is a circumstance. These straight nautiloid shells 

 we call Or-tho-cer'-a-tites (the technical name of the genus 

 being Or-ikoc'-e-ras or "straight horn"). Like Nautilus, the 



