JURASSIC, OR 
S3 
AMMONITES JUBAR.-<S7r«f/<ey, MSS. 
Plate 20; figs. 2 a-c. Plate 21 figs. 1 ci-c. 
A. testa discoidea, compressa. Anfractibus complanatis, costatis, 
intervalliui sulcatis, et apud peripheriam tuberculis binis depressis 
notatis. Costis flexuosis, bifidis vel 3-4 furcatis, ad marginem sub- 
obsoletis. Ventre planato. Apertura truncata-elliptica. Septis 
lobis lateralibus 4. 
Diameter . . . 3'3 in. (approx.) Spiral ratio . . » .5:2 
Ditto of outer whorl . 1 3 „ „ Immersion of wliorls , . £ 
Thickness . . 0-9 „ „ Thickness : diameter « . f 
This is a closely ribbed flat-whorled Ammonite; bearing a 
very near resemblance to A. Bakeria , Sow., from which it 
is distinguished only by the greater width and flatness of its 
whorls, and by the bending forward of the ribs on each side 
of the siphuncle. The ribs are filiform and flexuous, varying 
in number from 45 to 55; mostly bifid; and in old specimens 
irregularly fasciculate. They bend forward considerably in 
the ventral region (in some specimens more than in others), 
and become obsolete on the outer or median line. The stages 
of growth are marked sometimes by sulci and strong oblique 
varices; and in all cases the ventral region bears several pairs 
of low flat tubercles; (one on each side,) which occur at reg¬ 
ular distances, the intervals varying in different specimens. 
The sutures resemble those of A. Parkinsoni, Sow., and 
A. Wallichii, Gray, but the ventral lobe is, relatively to the 
superior lateral, shorter than in the last-named species, while 
the superior lateral differs from that of both species in being 
symmetrically trifid. 
Yar. a multiradiatus. Strachey, MSS. 
This differs from the normal form in the greater number of 
its ribs (55), which are consequently more close set and fili¬ 
form. The variation probably occurs only in young shells. 
