42 PSEUDOCERATITES OF THE CRETACEOUS. 
characteristic, having simply digitated or phylliform, often entire or bifid 
saddles and narrow, often club-shaped, lobes digitated at their tops. 
The first lateral saddles are usually bifid, but in some forms they 
become trifid. In these cases the additional saddles are added on the 
ventral side through the division of the outer arm of the first lateral 
saddle. The group as here defined is confined to the Lower Senonian in 
Europe, and the South American representatives are probably of about the 
same age. 
TISSOTIA Douvillé. 
The definitions heretofore given of this genus have been based upon 
the sutures which, according to the views here advanced, can not be accepted 
when not correlatable with external characters. The type of this genus, 
T. tissoti, so far as can be seen from the side view given by Bayle and the 
front view by Douvillé, is unique in having a considerably inflated form 
like some species of Stbtissotia, but with keel and ventral channels bordered 
by continuous ridges, and in the extreme gerontic substage a truncated 
venter without either keel or channels. This certainly justifies the opinion 
of Grossouvre that this species is closely related to Pseudotissotia galliennei. 
According to the views here advocated, this genus is a direct derivative of 
galliennei of the Turonian, as has been previously suggested by Peron. 
Tissoria TissoTi (Bayle). 
pl. 12, fig. 3. 
The type of Bayle’s species, which is also the type of his genus 
Tissotia, has been figured by him in side view and by Douvillé in front, 
from the same type in the Ecole des Mines, Paris. Douvillé also describes 
the specimen. It is therefore plain that it is a stout form without any very 
marked nodes and is flat on the venter in its gerontic stage, having lost its 
keel on the last part of the outer volution according to Bayle’s figure, 
which is natural size, and also Douville’s figure. This shell also, according 
to Douville’s figure, preserves an unusually depressed venter having keel 
and shallow channels on either side bordered by faint ridges or carinz 
throughout the adult stage. The loss of the keel is correlated with loss of 
the gibbosity of the sides, these becoming flatter aud more convergent 
outwardly. 
