392 Systematic Paleontology 



at nearly equal distances apart, between the first and the marginal row, 

 which forms the lateral keel. When more advanced in growth the sides 

 become rounded and convex; the dorsum proportionally wider and less 

 distinctly keeled ; the volutions somewhat more involved within the outer 

 one, which gives a correspondingly narrower umbilicus in proportion to 

 the entire diameter; the ridges crossing the sides are proportionally less 

 elevated and the nodes less conspicuous. In a large cast sent me, as one of 

 the type specimens, from the Academy of Natural Sciences, Philadelphia, 

 the thickness at the edge of the umbilicus is 2f in., when the width of the 

 volution is 3^ in. A small specimen, apparently entirely uncompressed, 

 presents a width on the side of the volution of three-eighths of an inch, 

 and a diameter of one-sixteenth less at the edge of the umbilicus. The 

 same features of the surface are present on both specimens, differing only 

 in degree. The septa are marked by three lobes and an imperfect fourth 

 one on the inner margin, and by three sinuses. The dorsal lobe has a pair 

 of short, principal, digitate branches, with several small digitations along 

 its sides. First lateral lobe moderately large, with four principal, much 

 serrated branches, and two or more minor ones on the neck. The second 

 lateral is irregularly branched, having two or three divisions, and the one 

 bordering the umbilicus has the margin simply undulated. The first 

 sinus is very large and divided in the middle by a long, slender, digitate, 

 minor lobe, which extends nearly or quite half the length of the dorsal 

 lobe. The second sinus is not more than two-thirds the size o"f the first 

 and far less distinctly divided. The small umbilical sinus has the margin 

 rather deeply undulated only. The margins of the sinuses are clavately 

 undulated and those of the lobes more sharply serrated; the number and 

 complication of these features varying, of course, with the size and age 

 of the shell. In the young specimens .... the complications of the 

 lobes and sinuses are more simple, although all the features are present." 

 —Whitfield, 1892. 



Type Locality. — Lower beds of Chesapeake and Delaware Canal. 



As Mortoniceras delawarensis is represented in Maryland by fragments, 

 there is little hope of determining its relationship to M. vanuxemi Morton. 

 Both forms occur in the Merchantville clay marl of the Matawan of New 



