DISTRIBUTION 95 



Of the 176 species, 109 are American and 67 European. 



It must always be remembered, in considering statistics of this kind, that 

 the numbers represent not the totals of the species as they actually existed in 

 the different epochs and areas, but only of the species which were preserved 

 and are represented in our collections. The infinitesimally small proportion 

 which the fossils obtained bear to the faunas which existed may be appreciated 

 when we reflect that of the life periods preceding the Ordovician, which some 

 geologists consider to have been longer than all those which followed it com- 

 bined, we have no representatives whatever except a few cystids; that vast 

 accumulations of fossiliferous strata have been obliterated by erosion during 

 periods of elevation of the sea bottom; that the strata covering three-fifths of 

 the earth are submerged beneath the oceans; that upon the land by far the 

 greater part of the fossiliferous rocks are buried under those of succeeding 

 formations, or under soil, ice, snow, or sand; that large areas are still unex- 

 plored; and that out of the strata which are accessible to us, all the fossils 

 which are obtained by our collectors represent only the partial product of not 

 more than a few, or a few hundred, acres. These facts are emphasized in 

 greater detail in Wachsmuth and Springer's monograph on the Camerata, 

 pp. 167-169. Therefore all generalizations based upon distribution must be 

 made with this qualification. 



Considering the known occurrences to represent in a remotely general way 

 an average of the succession of crinoidal life, we may note some facts indicated 

 by the above tables : 



The Flexibilia are very feebly represented in the Ordovician, from which 

 only two species of a single genus are as yet known, and of these specimens are 

 extremely rare. This is in marked contrast to the prevalence of crinoids in 

 the other orders, both Camerata and Inadunata being abundant and widely 

 distributed. There was a rapid increase in the Silurian to 71 species, chiefly 

 of the Niagaran or Wenlock period; followed by marked diminution to 34 

 species in the Devonian, and a revival toward Silurian abundance in the Car- 

 boniferous. Thus the two periods of maximum development in this group 

 were the Silurian and the Carboniferous; but these did not go on pari passu 

 as between the different families. The Lecanocrinidae show a rapid decline 

 from Silurian to Carboniferous, while the Taxocrinidae correspondingly in- 

 creased, so that the aggregate of the two is not very different in the two epochs. 



It is a singular fact that whereas the two known species of Flexibilia in 

 the Ordovician are highly typical Taxocrinidae, the great development of the 

 group which rapidly succeeded in the Silurian did not take place in this family 

 at all, but in the Lecanocrinidae, which in their typical forms are farthest re- 

 moved from it. This leads to the inference that the Lecanocrinidae developed 

 through a different line of ancestors in the Ordovician, or earlier. Their struc- 



