Marylaxd Geological Survey 541 



Genus PONTOCYPRIS Sars 

 POXTOCYHUS ARCUATA IL Sp. 



Plate XCVIII, Figs. 23-35 



Description.— Cara-paee moderately convex, subreniform, the dorsal 

 outline almost regularly arcuate, the ventral edge long, very slightly 

 sinuate; ends somewhat unequal, the posterior rounding sharply into the 

 ventral margin and very gradually into the dorsum, the anterior blunter ; 

 left valve overlapping the central part of the ventral edge of the riglit, 

 while the dorsal edge of the right similarly overlaps the left valve. In 

 edge views the outline is lenticular, tapering regularly and equally to each 

 extremity. In an end view the greatest thickness lies above the center. 

 Surface smooth. 



Length 1.0 mm. ; greatest height 0.56 mm. ; greatest thickness 0.38 mm. 



This species is related to P. smiihi and P. mawi, two Wenlock species 

 described by Jones. It is shorter than either of the British species and 

 the slight convexity of the ventral outline and the less acuminate posterior 

 end afford sufficient difference to distinguish it readily enough from P. 

 mawi. Critically compared with P. smitlii, which is the nearer of the two, 

 the present species is distinguished by its shorter form, more regularly 

 arcuate dorsum, straighter ventral edge, and in having the anterior and 

 posterior halves of the carapace less equal. 



Occurrence. — -Helderberg Formation, Keyser Member. Cumber- 

 land. 



Collection. — U. S. Xational Museum. 



PONTOCYPRIS MAWII VAR. BREVIATA JonCS 



Plate XCVIII, Figs. 20-28 



Pontocypris mawii var. ireviata Jones, 1889, Ann. and Mag. Nat. Hist. (6), 

 vol. iv, p. 269, pi. XV, fig. 4. 



Description. — -The specimens figured agree so well with the variety 

 hreviata of this well-known European Silurian species that the writers 

 have no hesitancy in so referring the American form. The shortening of 

 the anterior portion of the shell, thus producing a more equal ended 



