100 SYSTEMATIC PALEONTOLOGY 



oblique, most produced about the middle of the lower half; posterior 

 end narrower, the edge strongly curved, most produced about midway 

 of the height, with three or four small spines beneath this point; car- 

 dinal angles moderately distinct, subequal in the right valve but in the 

 left the posterior one probably more pronounced than the anterior and 

 more nearly terminal than in the right valve; lower half of anterior 

 margin with a row of small granules or spines. A well defined flat- 

 tened, un|)itted border encloses the ends and ventral side, very narrow 

 in front and below but gaining considerably in width as it passes around 

 the posterior end. Ventral edge very thick and nearly flat, the most 

 prominent portion of the surface being on this side. From this abrupt 

 ventral elevation, which is crowned by a row of blunt spines, the surface 

 descends gradually to the dorsal edge and more rapidly to the anterior 

 and posterior ventral margins. Surface markings consisting of large 

 pits and internodes, the latter increasing in size and prominence toward 

 the crest of the ventral ridge-like elevation. Pits generally occurring 

 in furrows but without any definite order of arrangement except on the 

 ventral edge where they are elongate and arranged longitudinally. 



Length of a right valve 1.15 mm., greatest height of same 0.60 mm., 

 greatest thickness of same 0.30 mm. 



In a side view this species resembles C. clarTcana greatly, but may be 

 distinguished at once by its flattened ventral edge. It is probably 

 more closely allied to. C. angulatopora (Eeuss) as figured by Bosquet, 

 but differs slightly in outline and surface markings, and in the greater 

 thickness of the ventral edge. In Eeuss' species the surface pits are 

 more regularly arranged in concentric series, and their are no nodes 

 betwe.en them, but there is a low submedian swelling, the like of which 

 has not been observed in C. planibasalis. 



Occurrence. — Chesapeake Group. James Eiver, Va. Probably also 

 in the Calvert or Choptank Formation in Maryland.' 



Collection. — U. S. National Museum. 



Cythere calverti n. sp. ? 

 Plate XXXVIII, Figs. 11-13. 



Comp. Cythere angulatopora (VReuss) Bosquet, 185:3, Desc. d. Entomostr. foss. d. 

 Terr. Tert. de la France et Belgique, p. 68, pi, ill, tigs. 5, a, b, c, d; also 

 Cythere uystiana Bosquet, op. cit., p. 65, pi. ill, tigs. 3, a, b, c, d. 



