280 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



the proposed monograph. While confessing its preliminary nature, 

 it should not be supposed that the results here presented are founded 

 on studies of only a part of the species immediately concerned. On 

 the contrary, the authors have taken into account not only every rec- 

 ognizably described or figured Beyrichian, but also a host of unpub- 

 lished species. If a classification of any family or subfamily could 

 be made final without first, or at the same time, subjecting all related 

 families to a similar close investigation, the following might lay 

 claim to being so. But as it is manifestly impossible to do this with- 

 out extending the field of study beyond the point attained, the present 

 contribution pretends to nothing better than a report of progress. 



ORIENTATION OF THE VALVES. 



The feature of the study of Paleozoic Ostracoda, about which the 

 literature of the subject seems to show greatest variability and un- 

 certainty among authors, is the determination of which of the two 

 cuds of the carapace and valves is the anterior. The rule most gen- 

 erally applied is to call the thicker end posterior. The present 

 writers are agreed with this as a general principle or rule, but not 

 as a law. Close comparisons, and especially exact analyses of the 

 lobes of Beyrichian forms, showed so many exceptions to the rule 

 that it seemed necessary to seek other and if possible more reliable 

 criteria. The position and trend of the median furrow was the first 

 feature to be investigated. Next the lobes were compared, and 

 finally the outline of the valves. It was found that all three of 

 these features afford more reliable evidence than does the relative 

 thickness of the ends. With the application of these several criteria 

 certainty and uniformity in orientation is attained, which, for pur- 

 poses of description and comparison, is, after all, the chief essential; 

 but there are no positive means, and perhaps never will be, of deter- 

 mining that the end of the fossil shell here called posterior did not 

 really lodge the cephalic organs of the living animal. Still the pro- 

 priety of the orientation adopted is supported by plausibility based 

 on facts, the bearing of which seems incontrovertible if not wholly 

 decisive. 



The principal line of evidence on which the orientation of the 

 valves of Beyrieliia and allied ostracods is based is derived from the 

 position of the eye tubercle and the outline of the valves of 

 Leperditiidae. That the small tubercle referred to was really con- 

 nected with the visual organs of the animal of Leperditia is univer- 

 sally accepted by paleontologists. Hence we are justified in assum- 

 ing that its location marks the anterior end of the carapace. Start- 

 ing with this accepted fact, we note (1) that the eye-bearing end of 

 the valve is almost always narrower than is the other end; (2) that 

 the outline of the valves exhibits a backward swing so that a rec- 



