MOLLUSCA. 831 



rounder and the nodes disappear; sides of the shell gently con- 

 vex from the umbilical shoulder to the venter, in young indi- 

 viduals with a row of distinct and somewhat sharp umbilical 

 nodes, and a second row of somewhat less prominent nodes about 

 one-third of the distance from the ventral to the umbilical mar- 

 gins; in the larger individuals these nodes disappear; aperture 

 elongate-saggitate in outline. Septa closely interlocking, the 

 lobes and saddles of comparatively small size but very compli- 

 cated, and varying with the growth of the shell ; the ventral lobe 

 is rather broad, passing straight across the venter except for 

 numerous minute crenulations, with a conspicuous lateral exten- 

 sion on each side; the first three lateral lobes are much larger 

 than the others and have from three to five main divisions be- 

 sides numerous finer serrations; at the base of the third lateral 

 lobe the general direction of the suture bends abruptly forward 

 to the top of the fourth lateral saddle, where it bends toward 

 the umbilicus again and continues to the dorsal margin ; beyond 

 the large third lateral lobe there are about six additional, more 

 or less bifid or trifid lobes which regularly decrease in size 

 towards the umbilicus ; the saddles are fully as complicated as the 

 lobes, but their general outlines are broader and rounder. 



The dimensions of a large individual are: diameter of shell, 

 222 mm. ; height of outer volution from venter to umbilical mar- 

 gin, 114 mm.; greatest thickness of outer volution, 50 mm. 



Remarks. This species, originally described from the Dela- 

 ware and Chesapeak Canal, occurs commonly in New Jersey in 

 the formations beneath the Navesink marl. A similar species 

 from the Upper Cretaceous of the West has usually been iden- 

 tified with the New Jersey form, but Hyatt has recently separ- 

 ated the western one under the name P. whitfieldi, restricting 

 the name P. placenta to certain forms from New Jersey and 

 Alabama. 1 In New Jersey the species reaches its greatest de- 

 velopment both numerically and as to size, in the Merchantville 

 clay-marl, in which formation as exposed at Reeves' clay pits at 

 Lenola, the most and best and largest examples have been col- 

 lected. The species also occurs in the Cliffwood clays beneath 



1 Monog. U. S. G. S., vol. 44, p. 221. 



