52 PSEUDOCERATITES OF THE CRETACEOUS. 
side of outer volution from line of involution to venter is 17 mm. The 
shell reduces the diameter of the umbilicus to 3 mm. 
The cast of the outer volution is perfect only on one side. This is 
planoconvex and covered with regularly arranged cost, of which there 
are eight to half of a volution on the inner part of the whorl. The costz 
broaden and flatten outwardly, becoming sigmoidal in curvature and then 
bifureate, the branches bending well forward and rising into elongated 
swellings on the cast. These are about 3 mm. from the outer edge of the 
keel, and there are very slight obsolescent ridges beyond them, which, 
however, do not cross the smooth bands on either side of the keel. 
The venter on the first part of the outer volution, the third quarter, is 
subacute, but on the last quarter it is beginning to be rounded, the keel 
having disappeared. This shows that the shell was in the gerontic stage, 
and the approximation of the last two sutures also indicates the same stage. 
There is also the trace of a line, probably a line of involution, impressed’ 
upon this part of the outer volution, showing that the living chamber must 
have decreased very rapidly in the amount of involution, since it did not 
cover quite half of the sides on the third quarter and part of the fourth 
quarter. of the now exposed volution. The length of the absent living 
chamber could not be determined, but that it was much rounded and 
depressed on the venter and in an extreme stage of gerontic degeneration 
is highly probable. The sutures are more like those of Paratissotia fourneli, 
as figured by Grossouvre in his Ammonites de la Craie supérieure. The 
ventral lobe is, however, very short, broad, and opens widely, with two 
small, short, entire arms. The siphonal saddle is broad, prominent, and 
entire, with a hardly perceptible median marginal lobe or depression, which 
seems inclined to disappear in the gerontic stage. The first lateral saddle 
is large, as in other species of this group. On the third quarter of the 
exposed volution it is plainly only one bifid saddle, but on the fourth 
quarter the dividing marginal is deeper, and there appear to be two dis- 
tinct saddles, as in P. delfensis. The outer arni of the saddle shows a 
hardly perceptible median marginal, as in P. fowrneli. There are five 
other lateral saddles, all entire, and, like the first lateral, very short 
and extremely broad like those of P. fichewri, as figured by Grossouvre. 
The first lateral lobe is broad at the end and denticulated exactly as in 
Tissotia tissoti and Metatissotia ewaldi as figured by Douvillé, but it is very 
short and broad. The second laterals are smaller and also denticulated. 
