446 CRETACEOUS PALEONTOLOGY. 



larva, but they are clearly distinct, and are for the most part 

 restricted to different geologic horizons, and in the New Jersey 

 faunas, at least, do not have intermediate connecting forms. 

 Their relationship to the true O. larva must be investigated. 



Formation and locality. Marshalltown clay-marl, near 

 Swedesboro (177); Navesink marl, near New Egypt (i47 2 ); 

 near Red Bank (120), Crosswicks Creek (195). 



Geographic distribution. New Jersey, Arkansas. 



Ostrea mesenterica Morton. 

 Plate XLIII-, Figs. 9-14. 



1834. Ostrea falcata var. B (0. mesenterica} Mort, Synop. 



Org. Rem. Cret. Gr. U. S., p. 51, pi. 9, fig. 7. 

 1861. Ostrea mesenterica Gabb, Synop. Moll. Cret. Form., p. 



209 (153). 

 1884. Ostrea (Alectryonia) larva White in (in part), 4th Ann, 



Rep. U. S. G. S., p. 296, pi. 42, figs. 6-7. 

 1886. Ostrea larva Whitf. (in part), Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 34, pi. 3, figs. 3-7. 







Description. Shell small, laterally arcuate. The dimensions, 

 of an average specimen are: length along the arcuate median 

 line from the beak to the posterior extremity 32 mm., distance 

 between beak and posterior extremity 26 mm. ; width of shell at 

 middle, 9 mm. Shell usually auriculate, the ears variable in size 

 and sometimes nearly obsolete, the posterior usually larger than 

 the anterior. Shell strongly plicate along its lower convex mar- 

 gin, the plications variable in number and size, not extending 

 into the median portion of the shell, those near the hinge-line 

 notably smaller than those in the middle of the shell ; the plica- 

 tions of the upper concave margin much smaller than those of 

 the convex margin. Central area of the shell marked only by 

 the concentric lines of growth. Lower valve moderately convex, 

 the scar of attachment variable, but usually small and restricted 

 to Jhe apical region ; upper valve flat. 



Remarks. This little oyster is extremely abundant in the 

 Navesink marl, and has not been observed in any of the beds 



