638 SYSTEMATIC PALEONTOLOGY 



above the hinge line, strongly incurved and nearly or quite in contact. 

 Antero-cardinal slope slightly concave or nearly straight ; anterior margin 

 rather sharply rounded ; basal margin broadly and regularly convex ; pos- 

 terior margin rather sharply rounded above to the posterior extremity of 

 the hinge line ; post-cardinal slope more concave than the anterior. Valves 

 ventricose or inflated in the umbonal region, the surface curving abruptly 

 to the cardinal margin, convex to the anterior and ventral margins, more 

 or less compressed to the postero-cardinal extremity; slightly gaping pos- 

 teriorly. Surface marked with forty or more angular, radiating costa? in 

 adult shells, the alternate ones being conspicuously larger. The smaller 

 costae are intercalated between the larger ones and do not reach the beak, 

 so that in very young shells the alternation of costae does not exist ; upon 

 the posterior, more or less compressed portion of the valves the costae are 

 nearly or quite obsolete. Distinct impressions of the external surface of 

 the shells show them to be marked by fine, indistinct lines of growth ; 

 they also show each costa, both the larger ones and the smaller ones, to be 

 surmounted by a row of fine tubercles or short spines, whose distance 

 apart is less than the spaces between the costse; the radiating lines of 

 tubercles are also present upon the posterior non-costate portion of the 

 shell. This species can be easily distinguished from /,. profe.rta by the 

 alternating costae and the more central position of the beaks. These two 

 species have never been observed associated in the same fauna, /,. aHernata 

 being characteristic of the Merchantville, while L. protexta is especially 

 characteristic of the Navesink." Weller, 1907. 



The occurrence in Maryland is restricted to a single broken valve, but 

 it is sufficient to show the diagnostic development of a secondary radial 

 lineation. 



Occurrence. MATAWAN FORMATION. Summit Bridge, Chesapeake 

 and Delaware Canal, Delaware. 



Collections. Maryland Geological Survey, New Jersey Geological Sur- 

 vey, U. S. National Museum. 



Outside Distribution. Matawan Formation. Merchantville clay marl, 

 New Jersey. Eutaw Formation (Tombigbee sand member). Exogyra 

 ponderosa zone, Mortoniceras subzone, Georgia : Perry County. Alabama. 



