LAMELLIBEANCHIATA OF THE LOWER MARLS. til 



^ny of the examples wliicli I have seen of this species. I do not think, 

 however, that there can be an}- doubt that the form here identified is that 

 which the author had before him when the description was written. There 

 is so nuich diversity also in tlie general form and expression of examples of 

 this type, when represented only by internal casts, owing to the greater or 

 less thickening of the valves with age and other peculiarities, that it is ex- 

 tremel}' difficult to draw the line of specific distinction between them; and 

 it becomes a serious question if nuxny of the species founded on these casts 

 may not be the same with forms described under other names from other 

 localities where the shell itself is preserved. This question, however, can 

 only be settled by obtaining impressions of the exterior from the beds where 

 the casts are found, by carefully made collections. Until this is done these 

 specific determinations will have to be accepted. 



The casts of this species have the general form and make-up of Con- 

 rad's genus Trigonarca, and if these divisions are to be accepted will neces- 

 sarily fall into that place. 



Formation and locality. — In the Lower Marls at Burlington, Mullica Hill, 

 and, perhaps, Arneytown, New Jersey; the latter being the locality men- 

 tioned with doubt by Mr. Gabb. 



Geuns OIBOTA Browne. 

 Cibota rosteUata. 



Plate XI, Figs. 34-36. 



Area rosteUata Morton. Synop., p. 64, PI. Ill, Fig. 11. 

 A. rosteUata (Mort.). Meek, Geol. Snrv. N. J., 1868, p. 725. 



Cibota rosteUata (Mort.). Gabb, Synop., 109. Meek, Smiths. Check-list, p. 9. Sto- 

 liczka, Pal. Indica, p. 346. 



Shell trapezoidal and very oblique, the length being about twice and 

 a half the height, with subparallel cardinal and basal margins, posterior end 

 very obliquely prolonged below, and the anterior end rather rapidly round- 

 ing backward from near the hinge-line to its junction with the base. Basal 

 margin very perceptibly sinuate nearly opposite the beaks and apparently 

 very slightly gaping. Valves moderately inflated, most ventricose on the 

 umbones just anterior to the sulcus, which crosses them from the beak to 

 the sinus of the base, and then rapidly declining to the anterior extremity, 



