M\l;^l.\\l) f;i;oi,()(;ic \ I, sri!\i:Y 173 



(liii;il Inoili --iiimII, I iiImt(IiI;ii-. Ilci<:lit. 1^ iiicli: |cii;^tli. I .^ iiidi."" 

 Coiiriul, isii I. 



This sjx-'cii's has a wide xcitical and hoi'izunlal raii;;(.' in the Maryhind 

 Eocene. It is hy iw means a ciinuiKiii tmiii, and j^ood spccimciis are 

 very rare. 



Cci'lain (lidl'-slatc Inrnis \\:\\r liccn ivt'cri'cd \n ||ii> species, hut the 

 n'ccnl woi'k of Di'. i)all sliows thai lln'V arc di^lind iind (hat iliis sjiecies 

 is appari'ntly rest ricti'd to ^Mai'yhand and \'ii'^inia. 



Lenglii. I'i nun.; width, 42 inni. 



Ocnirrnice. — Nanjemoy I-'oumatiox. Popes Creek, Woodstock. 

 Aquia Formation. Aquia Creek, Potomac Creek, 1 mile southeast of 

 Mason S])rings, 3 miles below Potomac Creek, Upper Marlljoro, Rolphs 

 Laiidini;', 1 mile northeast of Piscataway, mouth of Paspotansa Creek. 



CoUeciidiis. — Maryland Coolo^ieal Survey. Johns Hopkins I'niversity, 

 Philadelpliia Acadeiny of Natural Sciences. 



Family DIPLODONTIDAE. 



Genus DIPLODONTA Bronn. 

 DiPLODONTA MAELBOEOEXSIS n. Sp. 



Plate XXXVI, Fig. 4. 



Description. — Shell subcircular, moderately and regularly convex. 

 Beaks depressed, approximate. Lines of growth weak. Surface irregu- 

 larly wrinkled, sometimes smooth and almost polished. 



Differs from D. hopl-insensis Clark in being larger, more elevated, 

 much less globose, having a less prominent beak, and much weaker and 

 less frequent lines of growth. 



This species has a resemblance to D. uiitjulifera Conrad. The resem- 

 blance is especially close with Lea's figure of that species (described as 

 Egcria rotunda). 



Length, 18 mm.; width, 16 mm.; depth of valve, 4 mm. 



Occurrence. — Aquia Foematiox. Upper Marlboro. Sheckel's Farm 

 near South Eiver. 



Collection. — Marvland Geoloo-ical Survev. 



