MOLLUSCOIDEA. 367 



tening or shallow sinus occupied by from two to four plications. 

 Surface of the shell marked at intervals by rather strong, con- 

 centric lines of growth. Minute structure of the shell strongly 

 punctate. 



Remarks. This species has its closest relations with the 

 associated T. plicata, but it may be easily distinguished from that 

 species by its smaller size, its more elongate form, its more 

 numerous and fainter plications, and usually in the larger in- 

 dividuals by the more strongly convex brachial valve. Both 

 these species occur in the Navesink marl, and neither has been 

 found in any other horizon. T. vanuxemi is much the rarer 

 form, and in its typical form has not been observed to occur 

 actually in association with T. plicata, but in a bed several feet 

 beneath the horizon of that species. 



Formation and locality. Navesink marl, near Holmdel 

 (i28 3 ), Cross wicks Creek (195). 



Geographic distribution. New Jersey. 



