VM) 



THE AMERICAN NATURALIST [Vol. XLVII 



cation the bearing of these new genera from the Cam- 

 brian npon certain phases of marine biology, especially 

 on the probable age of the deep-sea fauna, led me to 

 examine all of them with the greatest care in order to 

 determine to my complete satisfaction whether the classi- 

 fication made by Dr. Walcott was beyond doubt justified 

 by the available facts. 



Dr. Clark and I examined the material together during 

 a visit which he made to Washington ; but we did not dis- 

 cuss the classification or the systematic position of the 

 genera. 



After the publication of Dr. Clark's article, as my ex- 

 amination of the material had led me to conclusions quite 

 different from those at which he had arrived, it seemed 

 advisable to put on record the results of my studies so 

 that those to whom the material is not accessible may 

 have, in addition to the published figures, which are won- 

 derfully good and leave little to be desired, the conclu- 

 sions of three entirely independent investigators, each 

 with a very different previous training, and each entirely 

 uninfluenced by the conclusions arrived at by the others. 



Before taking up the discussion of these forms in 

 detail it is advisable to give a brief outline of the general 

 principles of deduction by which my conclusions regard- 

 ing them have been reached. 



The characters by which animals are identified are of 

 two classes, the fundamental, or characters of prime sys- 

 tematic importance, and the correlative, or characters of 

 prime practical importance, though often of no system- 

 atic importance whatever. 



In the exceedingly rapid work, often under the most 

 unfavorable conditions, demanded in the identification of 

 organisms brought up by the dredge at sea one has no 

 chance to look for fundamental characters. The general 

 shape of the organisms, coupled with a few other obvious 

 features, alone are depended upon. Thus a shell with a 

 more or less polished surface and perfect bilateral sym- 

 metry is at once known to be a brachiopod, regardless of 



