570 CRETACEOUS PAI^EONTOLOGY. 



Swedesboro, and from these it has been learned that the hinge 

 is edentulous, a character which prohibits its reference tO' the 

 genus Sphaeriola. The hinge characters, as well as the general 

 form and characters of the shell, seem tO' agree in all essential 

 respects with members of 'the genus Unicardimn^ and the species 

 is therefore transferred tO' that genus. 



Formation and locality. — ^^Marshalltown clay-marl, near 

 Swedesboro (177) ; Navesink marl, Atlantic Highlands (108^),, 

 near New Egypt (147*). 



Geographic distribution. — New Jersey. 



Family LUOINIDAB. 



Genus Eucina Bruguiere. 



Lucina cretacea Whitfield. 



Plate LXIL, Pig. 18. 



1869. Conrad, Am. Jour. Conch., vol. 5, pi. 9, fig. 14. 

 1886. Lucina cretacea Whitf., Pal. N. J., vol. i (Monog. U, 

 S. G. S., vol. 9), p. 129, pi. 18, figs. 23-25. 



Description. — "Shell rather below a medium size, subcircu- 

 lar in outline, with the anterior and posterior margins forming a 

 very broad angle (140° toi 160°) with each other; giving in 

 some instances a nearly straight hinge-line, with the rather small, 

 pointed beaks slightly projecting beyond the line, and a slight 

 excavation in front, formed by the depression of the very narrow 

 lunule ; while behind the beaks the ligament is deeply sunken be- 

 tween the projecting shell margins. Surface of the shell marked, 

 by strong lamellose concentric lines, which are rather distant, 

 and where there has been no wearing or abrasion they some- 

 times roll backward, and form a hollow rounded concentric- 

 ridge. On the posterior side the shell is marked by a rather 

 deep constriction passing from the beak to- the posterior margin 

 just behind the umbonal ridge, or sometimes giving a sunken 

 postero-cardinal area. In the interior the muscular markings 

 are very deep and strong, and the hinge-plate narrow, the right 



