532 Systematic Paleontology 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, U. S. National Museum. 



Outside Distribution. — Matawan Formation. Merchautville clay marl, 

 New Jersey. Monmouth Formation. Navesink marl, New Jersey. 

 Ripley Formation. Exogyra costata zone, Eufala, Alabama; Pontotoc, 

 Union and Tippah counties, Mississippi; Lexington, Tennessee. Selma 

 Chalk formation. Exogyra costata zone, Wilcox County, and along the 

 Tombigbee Eiver, Alabama; east-central Mississippi. 



CUCULL^DA CAROLINENSIS (Gabb) 



Idonearca carolinensis Gabb, 1876, Proc. Acad. Nat. Sci., Phila., p. 314. 

 Idonearca carolinensis Boyle, 1893, Bull. U. S. Geol. Survey, No. 102, p. 152. 



Description. — " Shell subquadrate, convex, binge line just one-half the 

 length of the shell; beaks small, incurved, umbones prominent and 

 rounded ; posterior slope nearly vertical ; anterior end regularly rounded, 

 retreating obliquely below; base broadly convex, most prominent in the 

 middle. Surface in the adult marked only by irregular lines of growth; 

 in the young crossed by very numerous and very fine radiating lines; 

 hinge small. In the adult the middle (transverse) teeth show a tendency 

 to irregularity, and even partial obliteration. Lateral teeth perfectly 

 parallel with the hinge line ; area small. Internal plate thin and elevated. 

 Length 2 in. ; width 1.5 in. ; depth of single valves .75 in. This species 

 grows about the size of I. vulgaris, but is less oblique, with rounder out- 

 lines and a more central beak. The area is smaller, and the whole shell is 

 more quadrate. The markings of the young shell are as minute as those 

 of Trigonarca saffordi Gabb, but of a different character, and the present 

 species is proportionally shorter, more oblique, and more convex than that. 

 I. capax Conrad is a heavy shell, remarkably thick, and will, I think, prove 

 to be identical with vulgaris. I referred it to antrosa by mistake in the 

 Synopsis of Cret. Mollusca for that species. From I. neglecta this species 

 can be at once distinguished by the more convex valves and by the umbonal 

 angle. From the Eipley Group, Snow Hill, North Carolina.'' — Gabb, 

 1876. 



