MOLLUSCA. 689 



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

 wan (101), Lenola (163). 



Geographic distribution. New Jersey. 



Gyrodes petrosus (Morton). 

 Plate LXXVIL, Figs. 13-18. 



1834. Natica petrosa Mort., Synop. Org. Rem. Cret. Gr. U. S., 

 p. 48, pi. 19, fig. 6. 



1860. Natica alveata Con., Jour. Acad. Nat. Sci. Phil., 2d ser., 



vol. 4, p. 289, pi. 46, fig. 45. 



1 86 1. Gyrodes petrosa Gabb, Synop. Moll. Cret. Form., p. 117 



(61). 

 1864. Gyrodes alveata Meek, Check List. Inv. Foss. N. A., 



Cret. and Jur., p. 21. 

 1864. Gyrodes petrosa Meek, Check List Inv. Foss. N. A., Cret. 



and Jur., p. 21. 



1868. Gyrodes petrosus Con., Cook's Geol., N. J., p. 729. 

 1876. Gyrodes petrosa Gabb, Proc. Acad. Nat. Sci. Phil. 



(1876), p. 295. 

 1892. Gyrodes petrosus Whitf., Pal. N. J., vol. 2 (Monog. U. 



S. G. S., vol. 18), p. 127, pi. 16, figs. 1-4. 

 1905. Gyrodes petrosus Johns., Proc. Acad. Nat. Sci. Phil. 



(1905), p. 21. 



Description. "Shell (as seen in casts) of medium size or 

 smaller, obliquely oval or depressed and somewhat patulose, with 

 a low spire ; the entire adult shell having three to three and a half 

 volutions, the last of which forms the greatest bulk of the shell ; 

 volutions obliquely compressed from above, largest below the 

 middle, often slightly flattened on the supper half and with a 

 distinct flattened space bordering the suture ; aperture large, very 

 oblique, strongly receding below as seen in profile on its edge; 

 semilunate in outline, rounded below and slightly acute above, 

 somewhat modified in the upper part by the intrusion of the pre- 

 ceding volution; umbilicus large, broadly patulose within, and 

 apparently without callus; peristome thin, and the substance of 



44 PAI, 



