540 
of Trigonoceras are closely parallel. Ifthe adults were not known 
they would be referred necessarily to that genus. 
The group is of importance in the history of the impressed zone 
since it shows in its most specialized and highly involute members 
that a contact furrow may appear even in a form of whorl that has 
naturally a gibbous dorsum and concave abdomen. 
Fig. 2, Pl. xii, of Diorugoceras (aut.) planidorsatum (sp. Port- 
lock), Hyatt shows the peculiar character of the contact furrow in 
these forms when it occurs. 
It is probable that the early neanic. stage has a gibbous dorsum 
fitting into the hollow abdomen and that the involution is acquired 
rapidly in the later substages of the neanic stage, but not having 
seen specimens of the young I cannot state this as a fact. 
Triboloceratide. 
The figures of Zhoracoceras pusosianum of Pl. ix show a shell 
which in form isa slightly depressed oval and both in this respect 
and in the fluted ornamentation approximates to the nepionic stage 
of Zhoracoceras canaliculatum and other subspinous forms of the 
same genus. ‘This last species has also a similar form, and by com- 
paring this with the young of the loosely coiled, gyroceran forms 
on the same plate, Figs. 14 and 15 of ‘Triboloceras, it will be seen 
how closely they resemble them. Triboloceras in turn grades into 
the nautilian form of the same family, Vestinautilus Konincki, 
Figs. 5-13. The figures show the development of this form 
through a nepionic stage which is at first similar to Z. pusostanum, 
then becomes similar in ornamentation to 7. canaliculatum and 
then passing into the neanic stage these primitive characters are 
replaced by the peculiar acquired ornamentation and whorls having 
the hollow, central, ventral and lateral ventral zones of this family 
and smooth, gibbous, umbilical zones with broad, fluted, lateral 
faces. The subspinous ornamentation persists in this form on the 
ridges throughout the ephebic stage. In the gerontic stage these 
progressive characters disappear and with them the fluted faces and 
zones also tend to extinction and in the paragerontic stage do actu- 
ally give way to a rounded form without salient angles. This last 
is not figured, but the tendencies towards extinction of the orna- 
ments, etc., may be seen in the anagerontic substage delineated in 
Figs. 5 and 6. 
