STRATIGRAPHIC SIGNIFICANCE OF OSTRACODA 277 



Paleozoic rocks and the Cytheridse of Mesozoic and Eecent times that are 

 marked by great diversity of surface pattern, which lends itself to accu- 

 racy of specific discrimination. 



Taking up the Paleozoic Ostracoda, we find that they have a distinct 

 advantage over practically all other organisms in their occurrence in all 

 kinds of rocks. While most abundant in limy sediments, they are also 

 exceedingly common in highly siliceous strata in all kinds of shales, and 

 even in relatively coarse, beach-worn sandstone. They are thus ubiquitous 

 in their distribution and indiscriminate in the kind of water and sedi- 

 ments. 



Many species supposed to be Ostracoda have been described from Cam- 

 brian rocks, but recent unpublished studies show that all of these are 

 bivalved ph3dlopods. The first unquestionable Ostracoda, a few species 

 of Leperditia, are found in the early Canadian rocks of west Tennessee, 

 Missouri, Arkansas, and Oklahoma. Since they are wholly unknown in 

 rocks of essentially the same age in the Appalachian region, it is inferred 

 that the class, like many other groups of Paleozoic organisms, originated 

 in or south of the Gulf of Mexico. It is only in the later stages of this 

 period that the class attained representation in the more northern region. 

 In the Ordovician a great expansion of the class occurred. The Leper- 

 ditiidas continue in full or increased strength, while the main families of 

 Paleozoic time are introduced. 



During the Middle Ordovician there seems to have been a shifting of 

 the Ostracoda from the southern seas to the northern. This was accom- 

 panied by a considerable change in type. Thus, while the Ostracoda of 

 the Stones Eiver and the succeeding Black River faunas, which are of 

 southern origin, consist almost entirely of Leperditiidse, the next succeed- 

 ing deposits from the Baltic region and the northern areas of North 

 America contain very few or none of these, but instead a considerable 

 development of the more primitive types of Beyrichiid^e. Further, all 

 types of Ostracoda are rare, except a few like the cosmopolitan genus 

 Eurychilina, in the rocks of Trenton age in the Mississippi and Appala- 

 chian valleys. The later Ordovician rocks contain a great influx of spe- 

 cies quite similar to the late Black Eiver forms as developed in the Baltic 

 region of Europe and in America north of Missouri. During this time, 

 then, the supply seems to have been derived by emigration from the 

 northern seas rather than directly from the southern. 



The earliest Silurian, Richmond formation, has the same generic repre- 

 sentation; indeed, this continues with little change through the middle 

 Silurian. 



