PLACENTICERATID &, 213 
less numerous than in some specimens of whitfieldi. These disappear in 
the ephebic stage, together with the lateral line of tubercles, and in some 
specimens the latter may be entirely absent as in young specimens figured 
by Whitfield. The large originals of Morton’s figure and of Whitfield’s 
show these figures to be approximately correct. The venter is in all stages 
broader than in true whitfieldi, but remains flat only through a small part 
of the gerontic stage. In two specimens, 365 mm. and 425 mm. in diameter, 
‘one Morton’s type, from cut of Delaware and Chesapeake Canal, the living 
chamber was practically complete and about one-half of a volution in 
length. The rounding of the venter begins in these on the still separate 
part of the second quarter of the outer volution. Near the aperture the 
venter is completely rounded and the decrease in the ventro-dorsal diameters 
between the lines of involution more marked than in whitfieldi, and the 
umbilici are consequently larger. The sutures vary from having very solid- 
looking saddles as in Whitfield’s figure to those with the first to third 
saddles almost indistinguishable from those of some varieties of whitfieldi. 
Upon the whole, however, it is safe to say that while the saddles of placenta 
may be as deeply undercut and the necks as thin as in whitfieldi, the basal 
parts are, perhaps, always less completely cut up by the marginals. These 
and the auxiliary saddles have, however, as a rule, a bifid aspect with a 
large median marginal deeply dividing them, and this seems to be a distine- 
tion of more importance, especially in the aspect of the auxiliaries. The 
proportions of the diameters are also quite different. In the ephebic stage 
the transverse diameter may be more or less than one-third of the diameter 
from line of involution to venter and in old age it may be more or less 
than one-half of this same diameter. A young specimen of diameter of 
about 60 to 65 mm. is in collection of the Academy of Sciences of Phila- 
delphia. In the neanic stage this cast had highly compressed smooth 
volutions as in whitfieldi, but the umbilicus is larger, the involution being 
somewhat less, apparently. The three lines of tubercles begin on the first 
part of the outer volution or just before, when the shell is about 45 mm. in 
diameter. At this stage the lobes and saddles are distinctly placenticeran. 
The saddles are all bifid, but the entire outlines have given place to com- 
pletely denticulated outlines over both lobes and saddles, even on the 
innermost auxiliaries in the older parts of this cast. The sutures at this 
age are very similar to those of syrtale and guadalupe. 
