DISTRIBUTION OF PALEOZOIC OSTRACODES 



767 



ostracodes have been found in a great va- 

 riety of types of sedimentary rocks, both of 

 fresh water and marine origin. 



In the Paleozoic the preponderance of 

 marine over fresh-water species is very 

 great, largely because of the paucity of con- 

 tinental sediments in the stratigraphic col- 

 umn until near the end of the era. There are 

 fewer than a half-dozen Paleozoic fresh-wa- 

 ter genera known to me, of which only one 

 genus, from the Dunkard of Pennsylvania 

 and West Virginia, has been published. A 

 number of unpublished new species have 

 been found in the fresh-water members of a 

 few of the Pennsylvanian sedimentary cy- 

 cles in Illinois. 



In marine beds the ostracodes are most 

 easily obtained from the calcareous shales 

 adjacent to, and in the thin shale breaks 

 within limestones, where they occur abun- 

 dantly. They are usually associated with 

 bryozoans, crinoid fragments, brachiopods, 

 and, in some cases, with conodonts and For- 

 aminifera. Rarely some scolecodonts are 

 found in the washed samples. 



Although they are probably as abundant 

 in some limestones as in the adjacent shales, 

 they are very much more difificult to extract 

 and prepare for study. However, thin slabs 

 of limestones often contain great numbers of 

 excellently preserved specimens on their 

 weathered surfaces. This is particularly true 

 of many of those described from the Ordo- 

 vician and Silurian formations. In many in- 

 stances the only specimens available must 

 be laboriously prepared from the matrix of 

 the massive limestone but in some cases 

 good specimens can be obtained by careful 

 crushing (not grinding) the limestone in an 

 iron mortar, with frequent sieving. Silicified 

 forms can, of course, be segregated by the 

 use of dilute hydrochloric acid to dissolve 

 the matrix. 



Sandstones and sandy or siliceous shales 

 furnish the leanest source of ostracodes. In 

 these rocks the specimens usually occur 

 widely scattered throughout the matrix, and 

 often are found only as moulds or casts, 

 which, except in rare instances, fail to pre- 

 serve the delicate and intricate surface fea- 

 tures which are necessary for accurate iden- 

 tification. These rocks are usually too hard 

 to be broken down by boiling, and the moulds 

 cannot be removed by any known means of 



separation, except reproduction by one of 

 several methods of artificial casts or squeezes. 



STRATIGRAPHIC DISTRIBUTION 



True ostracodes are found in rocks of all 

 ages from Lower Ordovician to Recent. Spe- 

 cies first described as ostracodes from the 

 Cambrian were later found to belong to the 

 Branchiopoda. The oldest known ostracode 

 species came from thor Beekmantown and 

 Chazy rocks of Tennessee, Missouri, Arkan- 

 sas, Oklahoma and New York. 



Ulrich and Bassler (1923) have concisely 

 traced the early history of the class and their 

 conclusions, except for minor details, are 

 still generally accepted in the light of our 

 present knowledge. It is reasonably certain 

 the Ostracoda originated in southern seas 

 during the Early Ordovician by the devel- 

 opment of the Leperditiidae from the Cam- 

 brian branchiopods. In Middle and Late 

 Ordovician most of the Paleozoic families 

 were introduced, accompanied by a decided 

 shifting to northern seas. This caused a de- 

 cided change of type, the Leperditiidae, 

 Aparchitidae, and Eurychilininae giving 

 place to the primitive types of the Beyrichi- 

 acea of Europe and northern North America. 

 All types, except the Leperditellidae and the 

 widespread genus Eurychilina, are rare in 

 the Trenton formations of the Mississippi 

 and Appalachian valleys, and they are al- 

 most absent from the succeeding Cincinnat- 

 ian series. The latter does contain a large 

 number of species similar to those from the 

 late Black River from America north of 

 Missouri and in the Baltic of Europe. 



The brood pouches of the females, rarely 

 developed in the Ordovician, become gen- 

 erally developed in the Silurian. Commonly 

 the Beyrichiidae and the Zygobolbidae, all 

 Eurychilininae, and a few Primitiidae, no- 

 tably BolhiholUa, possess brood pouches. 

 The common presence of this feature is a 

 reasonably positive index of Silurian age. 

 Such a pouch is retained in only a few De- 

 vonian forms, such as Treposella and Tetra- 

 sacculus. 



The Devonian shows marked changes 

 brought about by the almost complete dis- 

 appearance of the Leperditiidae and by the 

 appearance of new genera of the Beyrichi- 

 idae. The new genera Kirkhya, Octonaria, 

 Thlipsura, Paraparchites, Tetrasacculus, 



