180 PSEUDOCERATITES OF THE CRETACEOUS. 
bifid, the outer arms more or less acute at the outer angles of the bases, and 
the inner arm narrower and tongue-like. One of the marked characters 
of this genus is the absence of differentiation between the parts of 
the suture at later stages of growth. The principal lateral saddles and 
lobes can not in many specimens be distinguished from the so-called 
auxiliary laterals, and I have not attempted to do this in my descriptions. 
In some specimens, on the other hand, especially young ones, there seem 
to be plainly only three principal lateral saddles or lobes, but in others 
there seem to be four or even five. The metaneanic substage in one 
species showed the primary division of the primitive first lateral saddle into 
three, and in several specimens in a late neanic substage it seemed obvious 
that this subdivision was maintained and that the first lateral of the 
derivative or principal series became bifid late in the life of the shell. First 
lateral saddles are therefore morphologically double, even when they are 
so fully divided that they have to be considered as two saddles, as 
in Metengonoceras dumbli. In most species there is no difficulty in seeing 
‘this, but in about all of them the line between the three principal laterals 
and the auxiliaries can not be drawn, nor between the principal lobes and 
the auxiliaries. Here, however, as in Engonoceras, there are but three 
principal laterals, if the first lateral is properly defined as double or bifid. 
The subacute venters of the species in this genus have lead to confusion 
with Sphenodiscus and its allies. The ontogeny of the latter separates the two 
generically, but the latter might be considered an accelerated form of the 
same family, as stated above in the description of the family, but for the 
sutural characters which show that Sphenodiscus belongs in the Placenticeran 
stock. 
METENGONOCERAS INSCRIPTUM n. sp. Hyatt. 
Pl. XXV, figs. 5-9; Pl. XXVI, figs. 1-4. 
One entire cast is 80 mm. in diameter. The diameter from line of 
involution to venter is 42 mm. at largest part, the transverse being 16 mm. 
Both measurements are a trifle short of what they would be in a more 
perfect cast. Opposite this the same diameter is 29 mm. without shell, 
the umbilicus being 9 mm. 
A fragment somewhat younger from same locality was not compressed 
so as to destroy the shape. This had the same form, but was not so con- 
cave near the umbilicus and had no folds. The volutions were flat on the 
