338 NORTH AMERICAN INDEX FOSSILS. 



6f. Without hook-like projection XLVII. Cypris. 



ooo. Valves not overlapping .jj, 



jj. Ovoid, with anterodorsal hook-like projection.,.. XLII. Cypridina. 

 jj. Elongate, bean-shaped, without hook or notch L. Cytherideh. 



5. Surface with median vertical furrow or depression e. 



i. Depression strong, a narrow sulcus present XLI. Entomis. 



i. Depression faint and indefinite III. 



III. Dorsal view triangular, hinge with a single posterior tooth. 



XLIX. Metacypris. 



III. Dorsal view ovoid or oblong L. Cytherideis. 



II. Valves with teeth and sockets along hinge-line, surface generally nodose or 

 pustulose 6, 



6. A single tooth present at each end of the hinge-line j 



j. Ventral margin not projected laterally 222 



222. Teeth connected by horizontal bar LI. Cyihere. 



222. Teeth not connected by bar LIA. Cythereis. 



j. Ventral margin with lateral wing-like projection LIII. Cytheropteron, 



6. Hinge-line with a row of small teeth LII. Cytheridea 



6. A single posterior tooth, laminated anterior projection XLIX. Metacypris. 



I. HiPPONiCHARioN Matthew. 



Valves subequal, wide, semielliptical and subequilateral ; free 

 margin with two prominent marginal ridges converging but not 

 meeting at ventral border. Central area greatly depressed, with 

 an inconspicuous central ridge near hinge line. Cambric. 



(This and the next genus are doubtfully referred to the 

 Ostracoda.) 



1. H. cavatum Matthew. (Fig. 1652, a, &.) Cambric. 

 Lateral ridges strong, nearly meeting in center of valve ; median 



ridge fainter, slightly sigmoidal. 



Middle Cambric, Protolenus beds of New Brunswick. 



2. H. minus Matthew. (Fig. 1652, c, d) Cambric. 

 Lateral ridges fainter, median ridge shorter than in preceding. 

 Occurs with preceding. 



IL Beyrichona Matthew. 



Subtriangular, with a rudely semicircular depressed area below 

 hinge, covering less than half the height of the valve. Cambric. 



(This and the preceding genus are doubtfully referred to the 

 Ostracoda.) 



