MOLLUSCA. 483 



tween. These are usually distributed in the following manner: 

 three between the two anterior strong ones; three or four be- 

 tween the two antero-basal and median ones; three usually be- 

 tween the postero-basal pair, and usually two only between the 

 posterior pair. There are also three or four anterior to the first 

 strong ray, and from four to six on the area posterior to the 

 last principal ray. The auriculations are also rayed, unequally 

 on the opposite sides, the posterior one most strongly. The 

 characters of the flat or left valve have not been observed on the 

 New Jersey specimens. The casts, the only condition in which 

 I have seen them from within the State, show evidence of mod- 

 erately strong concentric lines crossing the rays and intermediate 

 portions of the shell." (Whitfield.) 



Remarks. The secondary ribs between the six larger ones 

 shows considerable variation in the different individuals of this 

 species, but Whitfield was doubtless correct in his reference of all 

 the specimens to a single species. In 1850 D'Orbigny proposed 

 the specific name mortoni for this American form and was fol- 

 lowed by several authors, but that species seems to have been 

 founded upon insufficient characters, and in this place we fol- 

 low Whitfield in considering the American specimens to be 

 identical with the common European one. 



The species occurs at various horizons in the New Jersey Cret- 

 aceous, but is especially abundant in the Marshalltown clay-marl 

 near Swedesboro, where excellent specimens with the shells pre- 

 served occur. The shells do not grow so large, however, in the 

 Swedesboro locality, as the example illustrated by Whitfield, it 

 being a rare occurrence to collect a shell exceeding 40 mm. in 

 length. Some incomplete specimens of the flat valve from the 

 Navesink marl on Crosswicks Creek north of New Egypt, how- 

 ever, must have been fully as large as the larger specimen illus- 

 trated by Whitfield, 65 mm. in length. The Merchantville clay- 

 marl specimens more nearly agree with the Swedesboro specimens 

 in size. 



Formation and locality. Merchantville clay-marl, near Mat- 

 awan (101), Lenola (163), Burlington (Whitfield); Marshall- 

 town clay-marl, near Swedesboro (177, 179, 180) ; Navesink 



