254 GEOLOGICAL BIOLOGY. 



to open up the immense field of investigation which is here 

 suggested, the author's purpose in writing this book will be 

 fully rewarded. 



Brachiopods Thoroughly Differentiated in Early Paleozoic Time. 

 — When we critically examine a group of organisms like the 

 Brachiopods in their historical relations, we find a law of 

 successive appearance in geologic time of new characters, but 

 we are obliged to consider them minutely in order to under- 

 stand what is the nature of the evolution. The more impor- 

 tant characters were already present at the earliest period in 

 which records are preserved. 



Both of the orders of Brachiopods (Lyopomata and Ar- 

 thropomata) appeared in the Cambrian, and they are repre- 

 sented by numerous individuals and genera; and, according 

 to Waagen, there are three well-defined suborders of the 

 Lyopomata, and all of these were expressed certainly as early 

 as the base of the Silurian. If we take a later tabulation of 

 the genera and classification of Brachiopods,* we find ii 

 families of the Lyopomata with 55 genera, and 19 families -[- 

 14 additional divisions recognized as of subfamily rank, with 

 220 recognized genera belonging to the suborder Arthropo- 

 mata. Of the total 275 genera, recorded by Schuchert, 50 

 of the 55 Lyopomata and 139 of the 220 genera of the 

 Arthropomata, or 189 of the total 275, i.e. about 68 per cent, 

 appeared in Paleozoic time; and 17 genera of Lyopomata 

 and 5 genera of Arthropomata are already known in rocks 

 as ancient as the Cambrian system. 



Many of these Extinct since Paleozoic Time. — These figures 

 will give an idea of the great antiquity of the Brachiopods, 

 and of the early elaboration of the differences which are ex- 

 pressed in the systematic classification into genera. Another 

 fact can be expressed in mathematical form : not only were 

 the Brachiopods greatly evolved in early geological time, but 

 many of them have become extinct. Of the 189 genera of 

 the Paleozoic time 7 lived on to Mesozoic time, and of these 

 at least 2 genera still live. 



Generic Life-periods of the Brachiopods. — Again, although 



* Viz., that of Schuchert, in the American Geologist, vol. XI., March, 1893, 

 p. i.M, etc. 



