2 PEOCEEDINGS OF THE NATIONAL MUSEUM vol.70 



with the resulting frequent and often considerable change in sea 

 depth which there is ample evidence to show took place during the 

 upbuilding of these strata. As in the seas of the present day we 

 have littoral, off-shore and deep-water species wdiose range is largely 

 determined by sea depth, and still other species which have the ex- 

 ceptional power of adapting themselves to a very considerable 

 change in depth and other conditions, so in the ancient seas, life 

 appears to have been subjected to the same laws of distribution. 

 Then, as now, some species were evidently restricted to waters of so 

 little depth that they w^ere exposed at all times to the full force of 

 the tides and waves; others to seas though always comparatively 

 shallow during the period, yet of such depth that the agitation of 

 their surfaces seldom if ever extended down to their floors or dis- 

 turbed objects reposing upon them. In these shallow seas the re- 

 mains of only such forms as possessed a skeleton sufficiently compact 

 to withstand a long continued hammering in the surf after death, 

 came through intact, as for instance the bryozoans, the thick-shelled 

 species of brachiopods and gastropods and the true corals, while on 

 the other hand the shells of all the frailer forms after the death of 

 their occupants, were broken into fragments and these fragments 

 were usually worn smooth and scattered widely about by the waves 

 before the process of attrition was brought to an end by their being 

 finally covered up in the mass of debris accumulating on the sea 

 floor. It can be further said of these shallow sea deposits that the 

 shells of such bivalves as escaped destruction usually present the 

 evidences of erosion and as a rule have their valves separated and 

 lying widely apart. Also in those beds, species whose skeletons were 

 made up of a multitude of segments, as the trilobites, the crinoids, 

 and the starfishes, have almost without exception had these cover- 

 ings separated after death into their component parts and these 

 parts scattered by the waves. This statement is especially true of 

 some species of the genus Acidaspis whose range appears to have 

 been restricted to very shallow waters. Although abundant during 

 the Richmond, the constant turmoil of the waters in which they 

 lived and died, left only these fragments to be preserved in the 

 rocks. The same generally is true of the genus Lichenocrimis where 

 only the solid modified root remains. Although it is generally con- 

 ceded that the Richmond seas at no time attained abyssal depths 

 and that all times its waters were comparatively shallow, there are 

 good reasons for believing that not only once but many times during 

 that period its waters deepened to such an extent their floors were no 

 longer much affected by currents set in motion by forces acting upon 

 their surfaces. As evidence of this we meet with beds usually of 

 shale or marl in which the most delicate forms have been preserved 



