MAJIYLAXD GEOLOGICAL SURVEY G51 



C. vadosus Morton shows a wide range of variation in age characters. 

 The young are thin, rather compressed and truncated but not produced 

 posteriorly ; with increasing age the form becomes apparently more inflated 

 because of the umbonal thickening, and obliquely produced posteriorly. 

 (Plate XXXII, Fig. 3.) 



Conrad's C. ripleyana is doubtless a synonym, which includes the larger 

 and heavier individuals. The young are quite uniform in outline and 

 sculpture, but after the form has passed the typical C. vadosus stage there 

 is a strong tendency for it to become produced posteriorly and to develop 

 a rather heavy carina with the concomitant medial depression stage rep- 

 resented by the (7. ripleyana. The species differs constantly from C. sub- 

 planus in the heavier, less compressed and more inequilateral shell, the 

 less prominnent keel and the much heavier and more trigonal hinge plate. 



Although the species has not been reported from New Jersey it would 

 be by no means surprising if the numerous casts from the Monmouth, 

 which have been referred to C. subplanus Conrad, would find their true 

 affinities with C. vadosus Morton, which is by far the most abundant rep- 

 resentative of the genus in Maryland and constitutes, indeed, one of the 

 major factors in the Monmouth fauna. 



Occurrence. MOXMOUTH FORMATIOX. ? Millersville, Anne Arundel 

 County. Brightseat, railroad cut west of Seat Pleasant, Brooks estate near 

 Seat Pleasant, Fort Washington, Prince George's County. 



Collections. Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. Monmouth Formation. Navesink marl, and 

 Tinton beds, New Jersey. Ripley Formation. Exogyra costata zone, 

 Union and Tippah counties, Mississippi. Selma Formation. Exogyra 

 costata zone, Wilcox County, Alabama; east-central Mississippi. 



CRASSATELLITES SUBPLAXUS (Conrad) Johnson 



Crassatella subplana Conrad, 1853, Jour. Acad. Nat. Sci.. Phila., 2d ser., 



vol. ii, p. 274, pi. xxiv, fig. 9. 

 Crassatella subplana Meek, 1864, Check. List Inv. Foss., N. A. Cret. and 



Jur., p. 11. 



