252 PALEONTOLOGY OF NEW JERSEY. 



Ammonites Delawarensis. 



Plate XLii, Fig. 6-9 ; and Plate XLiii, Figs. 1, 2. 



Ammonites Delawarensis Morton: Am. Jour, of Science, 1st ser., vol. 18, PI. ii, 

 Fig. 4 ; Synopsis, p. 37, PL 2, Fig. 5 ; Gabb, Synopsis, p. 9 ; Meek, Check 

 List Cret., p. 24; Geol. Snrv. N. J., 1868, p. 730. 



Dr. Morton describes this species as follows: "Volutions uncertain; 

 each whorl furnished with elevated transverse ridges, which bifurcate about 

 half way across, and terminate in prominent tubercles on the margin; 

 ridges marked by three or four conspicuous nodes; back between the 

 tubercles convex; probable diameter from 8 to 12 inches." 



The shell seems to have been a very variable one, especially so when 

 different periods of growth are considered. The young form was described 

 by Dr. Morton as A. Vamixemi, in which condition it is somewhat discoid, 

 with a moderately large umbiliciTS with vertical sides ; about one-half only 

 of the volution being embraced by the succeeding one; the narrow dor- 

 sum being triply keeled; the marginal keels being formed of obliquely 

 elongated nodes formed by the extremities of the numerous, rounded costse 

 which cross the sides of the volution. A row of nodes marks the ends of 

 the costse along the margin of the iimbilicus, and three other lines occur 

 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 ele- 

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

 the type specimens, from the Acad. Nat. Sciences, Philadelphia, the thick- 

 ness at the edge of the umbilicus is 2§ inches, when the width of the volu- 

 tion is 3^ inches. A small specimen (figured on PI. xlii, Fig. 6), 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 speci- 

 mens, diff'ering only in degree. 



