522 



NORTH AMERICAN INDEX FOSSILS. 



growth lines, which are strongest on the posterior umbonal slope. 

 Ripleyan of New Jersey (Merchantville, Woodbury), Texas. 



438. M. saffordi Gabb. (Fig. 

 702.) Eocenic. 

 Umbo and umbonal ridge 



more prominent than in M. 

 alabamaensis. 



Midway: Tennessee, Texas. 



439. M. alabamaensis Aid- 

 rich. (Fig. 703, b.) 



Eocenic. 



Hinge margin slightly 



curved, ascending. Shell 



Fig. 703. a, Modioia [Brachydontes) strongly arcuate. Umbones 



multilinigera, left valve. (After Stanton. ) prominent Curved. 



Pamunkey : Maryland, Vir- 

 ginia. 



b, M. alabamaensis , right valve ; with indi 

 catorofsize. (Md. Survey 



CXVIII. Crenella Brown. 

 Shell small, ovoid. Beaks more or less incurved. Surface with 

 a thin epidermis and a fine radial striation. Cretacic-Recent. 



440. C. serica Conrad. Cretacic. 

 Radiating striae seen under a lens ; concentric lines large, fine 



and regular. Anterior slope from umbo abrupt. 



New Jersey (Marshalltown and Red Bank), Gulf region. 



441. C. elegantula Meek and Hayden. Cretacic. 

 Differs from C. serica in its much larger size, its proportionately 



broader form, and in the absence of conspicuous concentric 

 markings. 



Montanan : Nebraska-Montana ; Tinton of New Jersey. 



Order 2. ANOMALODESMACEA. 



CXIX. Pleuromya Agassiz. 



Slightly inequivalve. Posterior side the longer, somewhat 

 gaping. Hinge margin with a thin, horizontal lamina in each 

 valve, the left inferior ; the margin with a feeble notch behind the 

 lamina. Individuals of the same species quite variable. Triassic- 

 Comanchic. 



