MOLLUSCA. 575 



Remarks. The relations of this species to T. parilis have been 

 fully considered in connection with the discussion of that species. 

 It apparently differs from T. parilis chiefly in its smaller size, 

 its greater gibbosity and its stronger concentric lines of growth. 



Formation and locality. Navesink marl, near Walnford 

 (i 4 8 2 ). 



Geographic distribution. New Jersey. 



Super-family CARDIACEA. 



Family OABDIIDAE. 



Genus CARDIUM Linneus. 



Cardium nucleolus (WMtfield). 



Plate LXIIL, Figs. 8-9. 



1886. Criocardium nucleolus Whitf., Pal. -N. J., vol. i (Monog. 

 U. S. G. S., vol. 9), p. 214, pi. 28, figs. lo-n. 



Description. The dimensions of the type specimen are : height, 

 10 mm.; width, 10 mm.; thickness, 7.5 mm. Shell obscurely 

 quadrangular in outline. Hinge-line arcuate; anterior and basal 

 margins forming a single regular curve; postero-basal extremity 

 sharply rounded; posterior margin obliquely truncate. Beaks 

 nearly central, erect, moderately incurved and closely approxi- 

 mate. Valves ventricose with a subangular umbonal ridge from 

 the beak to the postero-basal margin ; posterior slope narrow 

 and abrupt; in front of the umbonal ridge the surface is regu- 

 larly curved to the anterior margin. Muscular impressions dis- 

 tinct and rather large on the casts; inner free margins of the 

 valves crenate. Surface characters and hinge unknown. 



Remarks. This is a peculiar, small, rotund form which is the 

 only representative of the genus yet observed from the Manasquan 

 marl. Whitfield referred the species to the subgenus Criocardium, 

 but in the absence of any knowledge of the surface features of 

 the shell this reference can only be considered as tentative. It 

 seems to be a rare form as it has not been met with in the recent 

 collections of the Survey. 



