MARYLAND GEOLOGICAL SURVEY 243 



Family RISSOID^. 



Genus RISSOA Freminville. 

 Subgenus ONOBA Adams. 



RissoA (Onoba) marylandica n. sp. 

 Plate LYIII, Fig. 5. 



Description. — Shell small, fragile, elongate, five-whorled ; whorls con- 

 vex; surface polished, with faint lines of growth and nnmerous fine re- 

 volving striag which are most distinct at the base of the whorl; mouth 

 wide; umbilicus distinct. 



The specimens from the Calvert formation differ f*rom the type which 

 is from Cove Point in having a more solid shell, finer revolving striae, 

 and a smaller umbilicus. 



Length, 4.5 mm. ; diameter, 2.5 mm. 



Occurrence. — St. Mary's Formation. Cove Point. Calvert For- 

 aiation. Plum Point, Church Hill, 3 miles west of Centerville. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 Cornell University. 



RiSSOA SP. 



A single fragment evidently represents a new species but is too imper- 

 fect to figure or to receive a name. The shell is elongate with very con- 

 vex whorls which are sculptured by distinct revolving ridges with broader 

 interspaces in which the lines of growth are very distinct. 



Length (of fragment), 2^ mm.; diameter, 1 mm. 



Occurrence. — Calvert Formation. 3 miles west of Centerville. 



Collection. — Maryland Geological Survey. 



Family ADEORBID/E. 



Genus ADEORBIS S. Wood. 



Adeorbis supranitidus S. Wood. 

 Plate LVIII, Figs. 6a, (3b, Gc. 



Adeorbis supra-idtidus S. Wood, 1843, Catalogue of Crag shells, Ann. and Mag. Nat. 



Hist., vol. ix, p. .530. 

 Adeorbis supra-nitidus S. Wood, 1848, Mollusca of the Crag, pt. i, p. 1.37, pi. xv, 



figs. .5a, 5b. 

 Adeorbis supranitidus Dall, 1892, Trans. Wagner Free Inst. Sci., vol. lii, pt. ii, 



p. 344. 



