CLARK : NEW GENERA OF CRINOIDS 605 



parentage. This fact, which is almost self evident, has been 

 verified by numerous analyses, which, however, have received 

 less attention than they deserve. 



Persilicic rocks, as I have already stated, are now the most 

 abundant plutonics and, in all probability, have been so from 

 the beginning. There is no reason for supposing that in this 

 respect any important change has occurred. Such rocks consist 

 mainly of quartz and feldspar, with only minor accessories. 

 Waters issuing from them are low in salinity but relatively 

 rich in silica and alkalies, the proportion of silica being especially 

 high and much in excess of lime. The silica often approaches 

 40 per cent of the total inorganic matter in solution, and in some 

 tropical rivers exceeds 50 per cent. Such waters offer a most 

 favorable environment for the growth of siliceous organisms, all 

 of which are low forms of life, like the radiolarians, diatoms, and 

 siliceous sponges. They, or their ancestors, were probably 

 among the earliest organisms to develop inorganic skeletons, and 

 were in greater abundance than the calcareous forms. Doubt- 

 less there were local areas, basaltic for example, in which the 

 waters carried much lime in solution, and here the conditions 

 would be reversed. In every case, however, the chemical char- 

 acter of the environment determined the chemical character of 

 the plants or animals which appeared. I speak now, of course, 

 only of those organisms which built skeletons to support their 

 tissues, or shells to house them; the simplest, earliest forms of 

 life were hardly more than aggregations of protoplasm. How 

 that originated and how it became endowed with life, that is, 

 the ability to move about, to assimilate food, and to reproduce 

 its kind, are questions on which only speculation is possible. 

 Such problems I must leave to ,others. 



ZOOLOGY. — Six new genera of unstalked crinoids belonging to 

 the families Thalassometridae and Charitometridae. 1 Austin 

 H. Clark, National Museum. 



A recent survey of the comatulid families Thalassometridae 

 and Charitometridae has shown that the following six system- 

 atic units are worthy of recognition as genera: 



1 Published with the permission of the Secretary of the Smithsonian Institu- 

 tion. 



