MOLLUSCA. 731 



outer half of the last volution ; the folds bent slightly backwards 

 from the suture to the center of the volution, and again very 

 faintly forward at that point, below which they rapidly become 

 obsolete, not showing on the under side of the volution." (Whit- 

 field.) 



Retnarks. This species occurs in the Navesink marl in the con- 

 dition of internal casts, and as in the case of most of the gastro- 

 pods known only as casts, its generic relations cannot be deter- 

 mined with certainty. The anterior canal has usually been 

 destroyed upon the specimens. The shells which Whitfield has 

 described under the name Tritonidea obesa prove, upon examina- 

 tion of the types, to be only small examples of Perissolax dubia. 



Formation and locality. Navesink marl, near Crawfords Cor- 

 ner (i26 7 ), Crosswicks Creek (195), Mullica Hill (169), Holm- 

 del (Whitfield). 



Geographic distribution. New Jersey. 



Perissolax trivolva Gabb. 

 Plate LXXXV., Fig. 6. 



1860. JPusus trivolvus Gabb, Proc. Acad. Nat. Sci. Phil. ( 1860), 



p. 94. 



1861. Perissolax trivolva Gabb, Synop. Moll. Cret. Form., p. 



122 (67). 

 1864. Perrisolax trivolva Meek, Check List Inv. Foss. N. A., 



Cret. and Jur., p. 23. 



1868. Perrisolaa trivolva Con., Cook's Geol. N. ]., p. 730. 

 1892. Perissolax trivolva Whitf., Pal. N. J., vol. 2 (Monog. U. 



S. G. $., vol. 18), p. 172, pi. 21, figs. 1-3. 

 1905. Perissolax trivolva Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 24. 



Description, "Shell of medium size, ventricose, with a long, 

 straight canal, once and a half as long as the length of the inner 

 part of the body whorl above it. Spire very low, broad, conical ; 

 the apical angle being from 100 to 110, the top of the volu- 

 tions flattened in the direction of the slope of the spire, and the 



