Maryland Geological Survey 535 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

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



Outside Distribution. — Matawan Formation. Merchantville clay marl. 

 New Jersey. Monmouth Formation. Navesink marl and Tinton beds, 

 New Jersey. Black Creek Formation. Exogyra ponderosa zone of 

 North and South Carolina. Peedee Sand. Exogyra cosiatu zone of North 

 and South Carolina. Ripley Formation. Exogyra costata zone. Extreme 

 top of zone, Union County, Mississippi. Selma Chalk. Exogyra costata 

 zone, Alabama Eiver, Alabama. 



Family ARCIDAE 



Genus ARCA Linne 

 [Systema Naturae, 1758, ed. x, p. 693] 



Type. — Area noo3 Linne. 



Shell equivalve, oval or subquadrate ; valves closed or gaping ventrally ; 

 sculpture generally heavy and radial; hinge line long, straight, furnished 

 with numerous transverse teeth : umbones prominent, incurved, prosogy- 

 rate, separated by a rhomboidal ligamental area scarred with cartilage 

 grooves ; margins smooth or crenulate ; pallial line simple ; adductor 

 impressions subequal, strongly marked. 



A genus represented by some five hundred fossil species ranging from 

 the Silurian onward, and by about one hundred and fifty recent species 

 widely distributed in the warmer seas from between tides to abysmal 

 depths. 



The abundance of the representatives of this genus, their wide range 

 in time and place, together with their rather more than normal sensitive- 

 ness to environmental conditions make them of peculiar importance in 

 stratigraphic work. 



A. Latitude of adult shell exceeding 12 mm.; umbones anterior. 



1. Altitude less than one-half the latitude, valves gaping ventrally. 



Area obesa 



2. Altitude approximately one-half the latitude; valves not gaping 



ventrally A >'<<' uandi 



B. Latitude of adult shell not exceeding 12 mm.; umbones submedial. 



Area saffordi 



Etymology: Area, a box. 



