MARYLAND GEOLOGICAL SURVEY 617 



smooth, excepting for the sharply laminar incremental ridges developed 

 near the dorsal and anterior margins and the prominent growth lines near 

 the ventral margins ; characters of interior not known. 



Dimensions. Altitude 10 mm., latitude 22 mm., maximum diameter 

 7 mm. 



This species is smaller than M. burlingtonensis Whitfield, not con- 

 stricted along the medial ventral margin, and more smoothly rounded 

 behind. 



Occurrence. MONMOUTH FORMATION. Brightseat, Prince George's 

 County. 



Collection. Maryland Geological Survey. 



Genus LITHOPHAGA Bolten 

 [Museum Boltenianum, 1788, p. 156] 



Type. Mytilus lithophagus Linne. 



Shell thin, nacreous, equivalve, strongly inequilateral, transversely elon- 

 gated, more or less cylindrical in outline ; umbones strongly anterior, but 

 not terminal ; anterior extremity rounded ; posterior extremity rostrate or 

 cuneiform ; external surface smooth or feebly sculptured concentrically ; 

 ligament submarginal; hinge edentulous; muscle impressions unequal, 

 indistinct. 



The genus has been reported from strata as far back as the Carbonifer- 

 ous. The recent species number less than fifty, and are confined to the 

 tropical and subtropical waters. 



The young are attached by a byssus, but in the later stages usually 

 perforate coral colonies, the shells of larger bivalves or even the solid rock. 

 Two of the five subgenera into which the group has been divided are 

 encrusted with a dense calcareous covering in the adult stages. The 

 cavities which they excavate are characteristically flask-shaped in outline. 

 The perforations in the columns of the temple of Serapis which served 

 Lyell for his classic illustration of changes in the level of the sea were 

 made by Lithophagae. 



Etymology: Aitfos, stone; <t>ayeit>, to eat. 



