182 SYSTEMATIC PALEO^^TOLOGY 



growth, and fine concentric lines, often obscure; lunules broad, deeply 

 depressed. 



" This species differs from C. alaeformis by its shorter, broader pos- 

 terior extremity; by the absence of deep, prominent furrows on the nm- 

 bones, and by the nmbones themselves being higher." Clark, 1896. 



This species shows considerable variation in size and form, sometimes 

 approaching the more elevate varieties of C. alaeformis.' On the other 

 hand it frequently becomes so short and elevate as to approach Astarte 

 in outline. This species is most abundant in the Piscataway substage. 



Length, 60 mm.; height, 50 mm. 



Occurrence. — Aquia Fokmation. Aquia Creek, Glymont, Eeedy Eun, 

 Brooks Estate near Seat Pleasant, Potomac Creek, Liverpool Point, Mat- 

 tawoman Creek. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 Philadelphia Academy of Natural Sciences. 



Ceassatellites alta (Conrad). 

 Plate XLII, Fig. 3. 



Crasmtella alta Gonrad, 1832, Fossil Shells of the Tertiary, No. 2, p. 21, pi. vii. 

 Grassatella alta Conrad, 1835, Trans. Geol. Soc. Penn., vol. i, p. 335. 



Descriftion. — " Suboval, thick and ponderous, compressed; anterior 

 margin obtusely rounded; posterior margin broad and slightly angular; 

 beaks with regular concentric grooves, and somewhat angulated behind; 

 inner margin crenulated." Conrad, 1832. 



Conrad recorded this species from Upper Marlboro in 1835, but there 

 is no other evidence of its occurrence at that locality. The large speci- 

 men here figured is from Hardesty and can belong to no other known 

 species. The large forms alluded to under G. alaeformis may be the 

 same as the Upper Marlboro forms which Conrad referred to C. alta. 



Length (of fragment), 105 mm.; width, 75 mm. 



Occurrence. — Aquia Foemation. Hardesty. 



Collection. — Maryland Geological Survey. 



