698 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



phylum or subdivision of it. Accordingly the more important advances made 

 during the past one hundred years have been centered on the study of special 

 groups, with particular attention to their geologic history. Many monographs 

 dealing strictly with the biologic aspects of faunas were, and still are being, 

 published but some of the more important, while contributing to the problems 

 of evolution, lay special emphasis on stratigraphic problems. 



The important advances made in the science of invertebrate paleontology 

 during the past one hundred years may best be outlined by the consideration of 

 each invertebrate phylum separately. 



Protozoa: The foraminifera, which in most cases have chambered tests, in 

 1798 were included by Cuvier in the Mollusca but later, when studied micro- 

 scopically by Dujardin in 1835, were found to contain protoplasm which was 

 free to circulate throughout the chambers, thus establishing their unicellular 

 structure. During the second quarter of the nineteenth century d'Orbigny de- 

 voted much time to a study of the Foraminifera and in 1826 made this group 

 of organisms a special order, which he included in the class Cephalopoda. The 

 order was divided into 52 genera and over 500 species based largely on the mor- 

 phological differences of the wall and the shape of the test. He later accepted 

 the interpretation of Dujardin that the Foraminifera were protozoans and in 

 1839 revised the earlier classification so as to consist of 6 orders and 64 genera. 

 His final paper in 1852 grouped the genera into 7 orders, in which the growth 

 and arrangement of chambers were of fundamental importance for classifica- 

 tion. He still adhered to the concept of immutalibity of species. 



The foraminiferal investigations initiated by d'Orbigny were continued by 

 numerous paleontologists, and many classifications were devised, based on dif- 

 ferent criteria as the description of new genera and species appeared in a 

 rapidly expanding literature. M. S. Schultze in 1854 proposed that all of the 

 foraminiferal genera be included in a single order Testacea, which he separated 

 from the rest of the unicellular organisms of the phylum Protozoa. The species 

 were included under 10 families and 112 genera, all classified according to the 

 shape of the test and arrangement of chambers. AVilliamson, who devoted spe- 

 cial attention to a detailed study of the structure of the hard parts, concluded 

 that the great differences shown in the structural characters rendered difficult 

 any exact classification. He regarded these differences as due largely to varia- 

 tion and arranged into groups those genera which appeared to be closely related 

 and grading into each other. A classification founded on the number, structure, 

 and arrangement of chambers of the test, as well as the perforate or imperforate 

 character of the wall, was proposed by A. E. Keuss in 1861. In addition, differ- 

 ences in the chemical and mineralogical composition were included. All of the 

 species were grouped into 21 families and 109 genera. 



H. B, Brady (1884), who studied the Eecent foraminifera obtained by the 

 "Challenger" Expedition, constructed a classification consisting of 10 families 

 and 153 genera based largely on the texture, shape and structure of the test, 

 aperture, and number and arrangement of the chambers. He was not in agree- 

 ment with Reuss, who had included all arenaceous foraminifera in the imper- 

 forate groups, because some of the genera possess mural pores. The Astrorhizi- 

 dae were considered by Neumayr (1887) as a primitive group and as the ances- 

 tors of the other foraminiferal families already established by Brady. On the 



