CRUSTACEA— OSTRACODA. 345 



X. Primitia Jones and Holl. 



Distinguished from Primitiella by having a well-marked sub- 

 central pit or sulcus instead of an undefined depression. As a 

 rule also the valves are shorter, the outline being generally more 

 ovate. Genotype P. mimdula Jones. At least fifty species are dis- 

 tinguished through the Paleozoic rocks. 



22. P. cincinnatiensis S. A. Miller. (Fig. 1658, /,/'.) Ordovicic. 

 Swollen behind the sulcus, with rounded node in front of it. 

 Upper half Cincinnati group of Cincinnati region. 



23. P. tumidula Ulrich. (Fig. 1658, m, m', n.) Ordovicic. 

 Sulcus shorter than in preceding, with faint node on either side. 

 Richmond of Minnesota. 



24. P. seminulum Jones. (Fig. 1660, J.) Devonic. 

 Antero-dorsal end rectangular; sulcus deep, central; surface 



reticulated. 



Hamilton of New York. 



XL Primitiopsis Jones. 



Oblong strongly convex shells with a long straight hinge line, a 

 faintly defined central spot, reticular ornament and a narrow con- 

 cave area on the inner side of the posterior edge. Genotype P. 

 planifrons Jones. Siluric-Devonic. 



25. P. punctulifera (Hall). (Fig. 1660, ^-^.) Devonic. 

 With three small, smooth tubercles, one central and two poste- 

 rior; reticulation obsolete at ends. 



Hamilton of New York, Ontario, etc. ; common. 



Xn. ^CHMiNA Jones and Holl. 



Primitia-like ostracoda, having instead of a sulcus a single, some- 

 times enormously developed, horn-like process. Genotype ^. 

 cnspidata J. and H. Eight species. Ordovicic-Devonic. 



26. A. spinosa (JHall). (Fig. 1659.) Siluric. 

 Spine strong, pointing upward, outward and forward, some- 

 times slightly bent; a rounded, thickened border surrounds free 

 margins of valves, sometimes pitted. 



Niagaran (Rochester shale) of New York, Ontario, etc. 



