CRUSTACEA— OSTRACOD A. 335 



1904. Ulrich, E. O., and Bass er, R. S. Miocenic Ostracoda of 

 Maryland and Virginia. Md. Geol. Survey, Miocene Volume. 

 The classification of the Ostracoda is still in an unsettled state. The 

 work of Messrs. Ulrich and Bassler among the Palaeozoic ostracods will, 

 it is expected, supply us with one, based on genetic principles, and the 

 student is referred to their papers as they appear. For later genera 

 the works of Jones and especially those of Brady on the recent species 

 should be consulted. The arrangement in the following pages is 

 chronologic so far as possible, no attempt at grouping into larger divi- 

 sions being made. 



Artificial Key to the Genera. 



A. Dorsal edge of valves straight, forming hinge-line I. 



I. Shell without sulci, nodes, ridges or prominent depressions i. 



I. Surface smooth, often glossy a. 



a. Valves regularly convex but unequal and overlapping oo. 



00. Left valve the larger, overlapping right valve III. Leperditella. 



00. Right valve the larger, overlapping left valve aa. 



aa. Shell large, generally with eye tubercle IV. Leperditia. 



aa. Shell small f. 



f . Without eye tubercle VI. Schmidtella. 



f . With eye tubercle ; ventral edge thickened. 



VIII. Paraparchites. 



a. Valves convex, equal, not overlapping il. 



II. Large, 3 mm. or more in length V. Isochilina. 



II. Small, less than 3 mm. in length VII, Aparchites. 



1. Surface pitted, striated or reticulated b. 



b. Hinge-line shorter than shell 22. 



22. With central or subcentral spot or pit ; valves subequal, not over- 

 lapping bb. 



bb. Spot faint, margin without rim XI. Primitiopsis . 



bb. Spot strong, subcentral, margin elevated XXXVII. Kirkbya. 



22. With excentric umbilical pit ; right valve overlapping the left. 



XXXVIII. Barychilina. 



b. Hinge-line forming greatest length of shell or nearly so. 



XVII. Macronotella. 

 I. Shell with broad dorsal depression 2. 



2. Depression subtriangular, defined by sharp converging ridges. 



I. Hipponicharion. 



2. Depression indefinite II. Beyrichona . 



I. Shell with one or more definite vertical or oblique grooves or sulci, with or 



without additional spines or nodes 3. 



3. With a single median or submedian depression or sulcus c 



c. With pronounced marginal flange 33. 



33. Flange continuous XVI. Eu^ychilina. 



33. Flange absent from anterior portion XXVIII. Ctenobolbina. 



c. Without marginal flange (a supramarginal sharp ridge may be more or 

 less developed) 44. 



