SNEAVER: INVERTEBRATE PALEONTOLOGY AND HISTORICAL GEOLOGY 703 



which earlier had been considered Bryozoa. Nicholson's (1886-1892) important 

 monograph on all known Stromatoporoids was published in the 'nineties and has 

 been followed, with many modifications, by later authors. 



The fossil ' Stromatoporoidea consist of large calcareous masses with great 

 variation in structure and shape. Their structure, based largely on fossils from 

 western Germany, has been investigated by M. Heinrich (1914), who divided these 

 organisms into two groups dependent on the massive or nonmassive character of 

 of the fibers. Those forms having a regular rectilinear arrangement of the fibers 

 were placed in the massive group and those with an irregular vermiculate ar- 

 rangement of the hollow-fibered skeletal structures in the nonmassive groups. 

 The principal characters used in both groups for distinguishing genera include 

 the amount of regularity and varying pattern of the skeletal mesh. 



An important contribution to the Anthozoa was published in 1900 by T. W. 

 Vaughan, with special emphasis on the general character and bathymetric distri- 

 bution of Eocene and Oligocene genera and species in North America. He consid- 

 ered that the classification of corals was in a very unsatisfactory condition, that 

 "no classification that will stand the test of thorough criticism has yet been pro- 

 posed . . . [and that] past classifications were based on some particular features 

 of the skeleton without reference to the whole structure and history of the organ- 

 ism." He pointed out that Duncan had based his grouping on a combination of 

 general skeletal features and mode of growth but had not searched for those char- 

 acters which were of phylogenetic importance. 



The accumulated evidence that the Tetracoralla were confined to the Paleozoic 

 and the Hexacoralla to later geologic time led P. E. Raymond to consider that the 

 cooler climate of Permian time had brought about the extinction of that group 

 largely because they had become specialized as lime-secreting organisms in rela- 

 tively high-temperature seas. He also proposed the idea that the Hexacoralla origi- 

 nated from an "Edivardsia-\ike actinian" of the Paleozoic and became a lime- 

 secreting organism as the temperatures lowered at the close of the Permian. Robin- 

 son suggested that certain forms classed as Tetracoralla occurring in the Meso- 

 zoic were probably Hexacoralla and that a number of Paleozoic genera referred 

 to the Hexacoralla were Tetracoralla. This pointed to a sharp distinction in 

 the time relations of these two groups. The paleozoic Cyathaxonia was sug- 

 gested as a specialized organism which might have evolved into Mesozoic types, 

 in which through assumption of an upright position the columella would change 

 from an excentric to a central location. 



The graptolites, which are abundant as colonies in black shales of the early 

 Paleozoic where they are compressed in the rock layers like fossil leaves, occur 

 as individual polyps attached to a central axis. They are important for distin- 

 guishing the geologic age of Ordovician and Silurian rocks and were placed by 

 many early authors, including Nicholson and Lapworth, with the Hydrozoans. 

 However, Neumayr held that they could not be placed with any of the known 

 classes of animals. In 1931 E. 0. Ulrich and R. Ruedemann, who had investigated 

 the graptolites for many years, discussed the question whether they should be 

 classed wdth the Hydrozoans or Bryozoans. Their objections to placing them with 

 the Hydrozoans were based on the ground that the Hydrozoa contain no structures 

 in common with graptolites, that they exhibit a different type of symmetry, and 

 that the graptolites include "a considerable number of unnaturally associated 



