MANTELLICERATID &. 107 
Other species of this genus are as follows: Pseudaspidoceras (Amm.) 
conciliatum, cunlifi, and perhaps crassitesta Stoliczka;* also Pseudasp. 
(Amm.) euomphalum Sharpe,’ and deciduum, equal Amm. rotomagensis 
@Orbigny.’ There is also a stouter species from the same locality as the 
last, Rouen, which has the ventral tubercles in close approximation and a 
much more aspidoceran aspect than in the usual form. This is Psewdasp. 
schlitert n. sp. Hyatt (Amm. rotomagensis Schliiter).” 
The fact that this species and Metacunthoplites rhotomagensis (Defrance) 
which has a line of median ventran tubercles and a distinct mode of devel- 
opment, have been so often published under the same specific name is very 
interesting. It shows how close is the parallelism of form between two 
widely separated genera, one in the group Cosmoceratida and the other in 
the Mammitida. 
DIADOCHOCERAS Hyatt.’ 
The young of this genus, first cited in Zittel’s Text-book, page 587, has 
in its youngest stages a more cylindrical volution and more discoidal form 
and slower growth than in Dowuilleiceras. The paranepionic stage has a 
coronate form with large tubercles and smooth sides, as seen from the 
umbilical aspect. At this stage the venter could not be seen. Later lateral 
costee appear and a line of very small tubercles at their inner ends. In 
the neanic stage there are ventral tubercles situated on the larger cost 
and a broad depression occurs between them on the continuous cost that 
cross the venter. This depression is not at first present in the intermediate 
untuberculated costze, which are continuous laterally and ventrally. Later 
in the ephebic stage the intermediate costa disappear on the sides but 
remain on the venter and become dichotomous from the lateral row of 
tubercles of the larger costations. This may be a matter of individual 
variation or it may be characteristic of different species, since in d’Orbigny’s 
figure (Terrain Crétacé, pl. 75) it is not present in the adult, and two speci- 
mens that I have handled of the type species, Diad. nodosocostatum d’Orb., 
differ, the costae remaining single in one and becoming dichotomous in the 
other. There is also a third row of tubercles developed in this genus on 
the umbilical shoulders that is not present in Dowvilleiceras until a later age, 
when the associated characters are quite distinct. 
«Foss. Ceph. Cret. Southern India, pls. 50-52. @Paleontogr., Vol. XXI, pl. 6. 
bFoss. Moll. of Chalk of England, pl. 13. € Ata oxos, a successor. 
eTerr. Crétacé, pl. 106. 
