528 CRETACEOUS PALEONTOLOGY. 



exclusive of the projecting beaks, subelliptical in outline. Beaks 

 central, or in some specimens apparently a little back of the 

 center, their apices pointed, elevated 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; 

 posterior 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, con- 

 vex to the anterior and ventral margins, more or less com- 

 pressed to the postero-cardinal extremity; slightly gaping pos- 

 teriorly. Surface marked with 40 or more angular, radiating 

 costag in adult shells, the alternate ones being conspicuously larger. 

 The smaller costse are intercalated between the larger ones and 

 do not reach the beak, so that in very young shells the alternation 

 of costse does not exist; upon the posterior, more or less com- 

 pressed portion of the valves the costse 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» pos- 

 terior non-costate portion of the shell. 



Remarks. — This species can be easily distinguished from L. 

 protexta by the alternating costse and the more central position 

 of the beaks. These two species have never been observed asso- 

 ciated in the same fauna, L alternafa being characteristic of the 

 Merchantville, while L. protexta is especially characteristic of the 

 Navesink. 



Formation and locality. — Merchantville clay-marl, near Ma- 

 tawan (100*, loi), near Jamesburg (139, 140). 



Geographic distribution. — New Jersey. 



