304 REPORT OF NATIONAL MUSEUM, 181)2. 



mentioned, and remarks in following paragraphs further explain the 

 known range of some of the subordinate, as well as that of the princi- 

 pal kinds. 



The horizontal spaces of the table represent the systems or stages of 

 the geological scale. The proportionate width of the spaces which 

 contain the names of those systems or stages is not intended to indi- 

 cate the actual ratio of geological time for each, but it may be stated 

 as the general opinion of competent investigators that the portion of 

 the scale from the Cambrian to the Carboniferous inclusive represents 

 a much greater length of time than does the portion from the Trias to 

 the Tertiary inclusive. In other words, it is generally believed that 

 the Paleozoic portion of the geological scale was of much longer dura- 

 tion than was that of the Mesozoic and Cenozoic portions together. 



The perpendicular lines in the table, which are placed singly or in 

 pairs or groups under letters of the alphabet from A to G inclusive, 

 represent the time range of the kinds of animals and plants which 

 have already been mentioned, and which for convenience of reference 

 are again recorded opposite corresponding letters at the foot of the 

 table. This method of grouping the different kinds of animals and 

 plants, as already intimated, is adopted only for present convenience 

 in making comparisons of chronological values. All the principal kinds 

 which are designated in the usual systematic classification are, how- 

 ever, included in these special groups, the few that are omitted being- 

 regarded as of little or no importance in this connection. The dotted 

 portion of certain of the lines indicates uncertainty as to the real ex- 

 tent of the time range which is shown by them because of imperfect or 

 doubtful representation of those kinds by discovered fossil remains. 



Of all the animals which have existed upon the earth whose remains 

 have been discovered only those of marine invertebrates have been 

 found to range through the whole geological scale. The time range of 

 these important portions of the animal kingdom is represented by the 

 group of five perpendicular lines under the letter A. The marine in- 

 vertebrate life thus represented includes the Protozoa, Coelenterata, 

 Annuloida. Annulosa. and Mollusca, the latter including the Mollus- 

 coida. That is, it includes rive of the six subkingdoms or branches of 

 the animal kingdom. 



The nonmarine and land invertebrates whose time range is intended 

 to be represented in the table by the two perpendicular lines under the 

 letter B are only insects and freshwater, brackish-water, and land 

 mollusks. The discovered fossil remains of all other nonmarine and 

 land invertebrates are regarded as either too rare or too unimportant to 

 be profitably considered in the comparisons which are to follow. The 

 longer of the two lines may be taken as representing the known time 

 range of insects and the shorter that of land and nonmarine mollusca. 



The pair of perpendicular lines in the table under the letter C shows 

 the approximate time range of all the various kinds of animal remains 



