MO'LLUSCA. 503 



tero-dorsal margin which is more or less carinate, posteriorly 

 the dorsal and ventral margins of the shell are subparallel, the 

 posterior margin rather narrowly rounded below, the antero- 

 ventral margin more or less concave. Adductor muscular im- 

 pressions rather large, situated near the posterior extremity of 

 the shell. Surface of the shell marked by fine radiating costse, 

 about three of which occupy the space of one millimeter, and 

 also by concentric lines of growth. 



Remarks. — This species is remarkable for the great convexity 

 of the valves, the thickness of the entire shell being considerably 

 greater than its width. Two complete specimens have been ob- 

 served, both of which show the enormous thickness of the shell, 

 although one is considerably more arcuate than the other. The 

 surface markings have not been satisfactorily determined because 

 the specimens are both for the most part internal casts, both of 

 them, however, retain some indications of the fine radiating 

 costae, and one of them retains a small fragment of the shell 

 itself. In the casts several concentric lines of growth are of con- 

 siderable strength. 



Pornmtion and locality. — Navesink marl, near Crawfords 

 Corners (126^), near Walnford (149). 



Geographic distribution. — New Jersey. 



Mytilus oblivius Whitfield. 

 Plate LV., Figs. 5-8- 



■ 1886. Mytilus oblivius Whitf., Pal. N. J., vol i (Monog. U. S- 

 G. S., vol. 9), p. 64, pi. 17, fig. I. 



Description. — "Shell small, erect, or but very slightly curved 

 on the buccal margin; beaks terminal, projecting and acute. 

 Hinge line sloping at an angle of about 60" to the buccal 

 margin; posterior margin subparallel to the anterior, and 

 the extremity rather sharply rounded. Anterior face abrupt, 

 and the surface of the valve gradually sloping from the umbonal 

 angle to the posterior margin. Surface apparently marked by 

 fine lines O'f growth as indicated on the cast." (Whitfield.) 



