MANTELLICERATID ®. 1S 
MANTELLICERAS n. gen. Hyatt. 
At first sight the type of this group, Mantelliceras mantelli, appears to 
belong to the same genus as Metoicoceras, but a slight examination of the 
development shows them to be generically separable. The tuberculated 
young Mantelliceras approximates to the type of development exhibited by 
Douvilleiceras, and is a compressed tachygenie form of the same family. 
The adult characteristics also show no close affinities for the Pulchellian 
group to which Metoicoceras probably belongs. 
The type species can not frequently be separated from Calycoceras “ 
navicularis, owing to the similarities caused by the continuity of the coste 
across the venter and the presence of similar lines of ventral and lateral 
tubercles. The development, however, and the forms in well-preserved 
fossils are both distinct and their variations lead in different directions. 
C. navicularis has the coronate form prolonged in its early stages and the 
coste prominent on the venter and a median ventran line of tubercles 
during its neanic stage and sometimes later. The furrowing of costa on 
the venter is of later development, and if Sharpe’s figures and descriptions 
in Fossil Mollusca of the Chalk of England, page 39, pl. 18, are correct, 
it is due to the disappearance of this line of tubercles. I have not been 
able to get the young of this species for study, but that this line of tubercles 
is sometimes retained until later is obvious in some specimens that I have 
had in hand. 
The relations of the young to those of Dowvilleiceras are obvious during 
the stage in which the volutions have a very broad ventral channel, with 
coste crossing the venter and six rows of tubercles, and before the 
innermost and eighth rows arise ou the inner ends of the cost. In 
the octotuberculate stage they are like the full-grown form of Sharpeiceras 
laticlavium. The shell of M. mantelli is at that time, a middle neanie 
substage, very closely similar to Douvilleiceras mammillatum in its sexitu- 
berculate stage. No additional lines of tubercles are added in this genus. 
On the other hand, the tendency is toward complete suppression of all 
except the two ventral rows. These disappear also in extreme age. ‘The 
sutures are obviously of the same type as in other genera of this family and 
are closer to those of Dowvilleiceras than any other genus. 
“« KaAvé, calyx. Noted in Zittel’s Text-book, Cephalopoda, p. 589. 
03 8 
MON XLIV 
