706 LOUIS AG A SSI Z. 



ages, similar types inhabited shallow seas? 

 Of the fact there can be no doubt, for it is 

 not difficult to adduce satisfactory evidence of 

 the shoal - like character of the Silurian de- 

 posits of the State of New York ; their hori- 

 zontal position, combined with the gradual 

 recession of the higher beds in a southerly 

 direction, leaves no doubt upon this point; 

 and in the case of the Jurassic formation al- 

 luded to above, the combination of the cri- 

 noids with fossils common upon coral reefs, 

 and their presence in atolls of that period, are 

 satisfactory proofs of my assertion. What 

 does it mean, then, when we find the Penta- 

 crinus and Rhizocrinus of the West Indies in 

 deep water only? It seems to me that there 

 is but one explanation of the fact, namely, that 

 in the progress of the earth's growth, we must 

 look for such a displacement of the conditions 

 favorable to the maintenance of certain lower 

 types, as may recall most fully the adaptations 

 of former ages. It was in this sense I alluded, 

 in my first letter to you, to the probability of 

 our finding in deeper water representatives of 

 earlier geological types ; and if my explana- 

 tion is correct, my anticipation is also fuUy 

 sustained. But do the deeper waters of the 

 present constitution of our globe really ap- 



