OOLITIC AMMONITES. 
85 
The ribs of this species terminate abruptly on each side of 
the siphonal region, leaving a smooth excavated space between. 
In very old specimens, they are, however, continuous across 
this area. The ribs are, for the most part, either simple or 
bifid, but, in some specimens, two or more spring together 
from the umbilical margin. The proportions of the whorls 
are somewhat variable, the breadth of the outer whorl rang¬ 
ing from yy to of the diameter, and the thickness from 
Tun YnV The ventral lobe of the sutures is rather shorter 
than the superior lateral, which is broad, open, and sub- 
symmetrical. 
A variety of this species is figured at plate 19, figs. 1 a-b, 
the ribs of which terminate ventrally in two rows of tubercles. 
The shell is much compressed, and the ribs numerous and 
regularly bifid. 
AMMONITES EOBUSTUS.-Sb-ac^y, MSS. 
Plate 16, figs. 1 a-c. 
A testa plus minusve globosa, valde costata, senectute. tuberculatd. An- 
fractibus amplectentibus, add umbilicum crassioribus, apud ventrem compres- 
siusculis, rotundatis: lateribus subplanatis. Costis validis, irregularibus, 
rursurn ac rursum bifurcatis, apud J dlametri anfractus intervallim tubercula- 
tis in medio ventre interruptis. Apertura semicirculari, subtrigona, vel. 
sagittata. Umbilico minimo. Lobis lateralibus 5. 
Diameter ... 
Ditto of outer whorls 
Thickness .. 
Spiral ratio ... 
Immersion of whorls 
Thickness : diameter 
< 1 * 
... 3T in. 
... 1‘8 „ 
... 16 „ 
b. c. 
2 2 in. 2 - 4 in. 
115 „ 
1-5 „ 08 
20 : 9 
A very variable form, as may be inferred from a comparison 
of the above measurements. It is sometimes much compressed, 
with a narrow, almost keeled ventral region;—sometimes 
globose. The ornament is characteristic. The ribs bifurcate 
repeatedly at various parts of the whorl, and are interrupted 
in the middle of the ventral region, leaving a narrow sulcus. 
