THE BRYOZOA — BASSLER. 377 



crustaceans are usually too scarce ; mollusca, abundant in some forma- 

 tions, are almost wanting in others, and likely to be poorly pre- 

 served; vertebrate remains are too few, and usually local in dis- 

 tribution. The brachiopods are also usually abundant in all Pale- 

 ozoic strata, but have commonly too great a range vertically to be 

 trusty guides in close work. 



In the Mesozoic rocks of America bryozoan faunas are few and so 

 far little known, but in Europe they assume an importance equally 

 as great as the Paleozoic faunas in America. In both continents the 

 Cenozoic faunas are abundant and of great value for correlative 

 purposes. In North America over 1,000 Cenozoic species are known, 

 while in Europe the number is equally large. 



Because to the unaided eye there seems little variation of form 

 among the bryozoa, they have been generally neglected by collectors 

 and geologists. Early writers are also to some extent responsible 

 for this neglect, for they failed to discriminate the different species, 

 and made a few names, such as Ckaetetes lycoperdon, Stenopora 

 fibrosa, etc., serve for a multitude of diverse forms. It is no doubt 

 true, and this is another cause for the neglect of the bryozoa, that 

 their discrimination does require good powers of observation and 

 careful, often tedious, study. Furthermore, the number of species is 

 great. Somewhat more than 1,500 species have been described from 

 American Paleozoic formations, yet these are probably but a half 

 or a third of the distinguishable forms present and already largely 

 known to specialists in the subject. The determination, at least the 

 first determination, of the species often, and among the Trepostomata 

 nearly always, requires the preparation of microscopic sections, a 

 tedious operation at best. However, when once a species has been 

 thoroughly worked out, it can generally be distinguished externally 

 from associated forms of similar appearance by quite constant dif- 

 ferences, which often seem trifling and yet are doubtless of mor- 

 phological importance. These various considerations would seem to 

 compel greater labor for the mastery of the bryozoa than for any 

 other class, but accurate determination of the brachiopods, corals, 

 graptolites, and other more widely studied groups requires equally 

 great efforts. 



In spite of numerous researches on the bryozoa as a whole, a be- 

 ginning only has been made in the work of determining the geo- 

 graphical distribution of species and genera and of elucidating the 

 many obscure questions regarding the migration of faunas in the 

 ancient as well as in the modern seas, their extinction or evolution, 

 their reapparition and like phenomena. Similarly the study of the 

 larval forms, the anatomy of the polypide, and of the various sub- 

 jects concerned in the relationship between the polypide and the 

 zooecium offers a wide field of research. 



