JURASSIC, OR 
80 
In the adult, every third or fourth rib is furnished with .a 
prominent tubercle, at about three-quarters the diameter of the 
whorl, where the ribs finally bifurcate. 
In the figures, two different specimens are represented. 
Fig. 1 b is tolerably correct, except that the ventral sulcus 
is exaggerated below. In fig. 1 a, the tubercles should be 
placed only at every third or fourth rib, and somewhat nearer 
to the umbilicus, and a bifurcation should be represented at 
every tubercle. 
AMMONITES GRIFFITHII- —Sbrachey, MSS . 
Plate 20, figs. 1 a-c. 
A. testa discoidea, compressa, complanata, late umbilicata, costata, 
apud ventrem seriebus 2 tuberculorum ornata, inter quce venter exca- 
vatus. Costis numerosis, irregulariter bifurcatis, apud ventrem 
continuis. Apertura ovato-oblonga. Septis lateraliter 3-lobatis. Lobo 
ventrali brevissimo. 
Diameter ... ... ... 4’7 in. Spiral ratio .. . 2:1 
Ditto of outer whorl ... 17 „ Immersion of whorls ... ... £ 
Thickness ... ... ... IT „ Thickness: diameter ... ... £ 
As represented in the figure, the two rows of tubercles 
which ornament the ventral region of this shell are seen co¬ 
vered up by the adhering shell of the succeeding whorl, and 
thus appear to be two keels, with a smooth intervening exca¬ 
vated area. Another part of the shell explains the real state 
of the ease, viz., that the ribs are continuous across the 
siphuncular region, and bear two rows of smaller tubercles, 
similar to those of A. Duncani , IF Orb., to which the present 
species is nearly allied, if indeed the two forms be not mere 
varieties of the same species. The ventral lobe is not more 
than half as long as the superior lateral, which last is as 
broad as the ventral saddle and terminally trifurcate. 
Only one specimen, viz., that represented in the figure, 
occurs in the Collection. It is an imperfect shell, and bears 
the remains of three-fourths of another whorl. The figure 
is three-fourths of the real size of the specimen. 
