MO'LLUSCA. 569 



Formation and locality. — Navesink marl, near Crawfords Cor- 

 ner (126''). 



Geographic distribution. — New Jersey. 



Super-family LUCINACEA. 



Family UNIOARDIIDAE. 



Genus Unicardium d'Orbigny. 



Unicardium umbonata (Whitfield). 



Plate I,XII., Figs. 16-17. 



1886. SpTueriola umbonata Whit., Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 152, pi. 19, figs. 17-18. 

 1905. Sphceriola imibonata Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 14. 



Description. — The dimensions of a large left valve are: 

 length, 57 mm.; height, 54 mm.; convexity, 18.5 mm. Shell 

 very thin, subcircular or slightly subquadrangular in outline. 

 Cardinal margin arcuate, edentulus; anterior margin broadly 

 rounded, its. most anterior point being at or below the middle, 

 rounding below intO' the basal margin; basal margin curving 

 upward at each end, straighter in the middle; posterior margin 

 usually a little shorter than the anterior, regularly rounded or 

 some times a little straightened, in the middle. Beaks strongly 

 incurved, pointing forward, situated at the middle or a little back 

 of the middle of the cardinal margin ; umbones prominent, much 

 elevated above the hinge-line. Valves strongly convex or ventri- 

 cose, the anterior slope somewhat more abrupt than the pos- 

 terior, slightly com]>ressed towards the ca'rdinal extremities. 

 Surface of the shell marked only by concentric lines of growth 

 which are more or less irregular in the streUigth of their develop- 

 ment. 



Remarks.— l^his species was described by Whitfield from a 

 very imperfect cast of the interior, and was referred to the genus 

 Sphaeriola without any knowledge of the hinge characters. In 

 the recent collections of the Survey specimens preserving the 

 shell have been secured from the Marshalltown clay-marl near 



