NKKLKSANDBAssLEK.] STKATIGKAPHIC USE OF BRYOZOA. 121 



GENERAI. :N^0TKS. 



VALUE OF BRYOZOA IN STRATIGRAPHIC WORK. 



The value of the biyozoa in stratigraphie work has scarceh' yet 

 beg-un to })G appreciated. In Anierican Paleozoic strata they are pre- 

 eminently the fossils to be relied upon in correlation work. They are 

 nearly always abundant, and even when poorly preserved exteriorly 

 can be identiiied by microscopic sections. Crinoids and crustacea are 

 usually too scarce; mollusca, abundant in some formations, are almost 

 wanting- in others, and likely to be poorly preserved; vertebrate re- 

 mains are too few, and usually local in distribution. The brachiopods 

 are also usually alnmdant in all Paleozoic strata, but have commonly 

 too great a range verticalh" to be trusty guides in close work. 



Because to the unaided eye there seems little variation of form 

 among the bryozoa, thej 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 Chwtetes lycoperdon^ Stenojmr a 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 discrimina- 

 tion does require good powers of observation and careful, often tedious, 

 study. Furthermore, the number of species is appalling. Somewhat 

 more than 1,300 species have been described from American Paleozoic 

 formations, j^et these are probably but a half or a third of the distin- 

 guishable forms present. These various considerations compel greater 

 labor for the mastery of the bryozoa than for any other class. The 

 determination — at least the first determination — of a 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 thoroughh' worked out, it can generall}^ be distin- 

 guished externally from associated forms of similar appearance h\ 

 quite constant differences, which often seem trifling, and yet are doubt- 

 less of morphological importance. 



A beginning onl}^ has been made in the work of determining the geo- 

 graphical distribution of species and genera and elucidating the many 

 obscure questions regarding the migration of faunas in the ancient seas, 

 their extinction or evolution, their reapparition, and like phenomena. 



COLLECTION AND STUDY. 



In sandstone formations bryozoa — and this is true of most other fos- 

 sils as well — are practically wanting, but there is scarcely a limestone 

 formation, especially if there be shale alternations, in which they are 

 not abundant. Generalh^ they are calcareous, and in this condition 

 are easil}' sectioned for microscopic study. Sometimes, however, they 

 are found silicitied. Then the internal structure is to a greater or less 



