SECOND, OR SCHLOTHEIMIAN BRANCH. ul 
their old proportions. In the full adult condition the characteristics of the 
sutures differ considerably from those of the typical Arietide, and approximate 
to those of Psiloceras. 
The seventh whorl increases in size with great rapidity, the abdomen be- 
coming narrower, the channel shallower, the pile more depressed, losing their 
prominent somewhat abrupt genicular bend, and on the abdomen becoming 
depressed to a level with the siphonal line. The involution of this whorl is 
about two fifths, and that of the ninth a trifle over one half. The peculiar flatten- 
ing of the sides and form of the adult whorl, and the amount of involution, are 
close approximations to the adult characteristics of Amm. Charmassei, but the 
septa are different and the young more robust; the pils are developed earlier 
and more rapidly, and the abdominal channel also. In some specimens, however, 
these last are not noticeable until quite a late period, the pile being continuous 
across the abdomen, as in Der. planicostum, even on the sixth volution. 
In the collections at the Stuttgardt Museum are several very fine specimens 
of the old age of this species, and it is easy to distinguish it from Charmassei by 
the narrowness of the whorls and its more open umbilicus and discoidal aspect. 
One of the largest of these measures 495 mm., the last whorl 17 mm.; another 
measures 515 mm., and the last whorl 18.5 mm. 
In the Museum at Stuttgardt, in the centre of a crushed specimen of the true 
angulata from Kirchheim, the young was very clearly exposed. This had very 
smooth and round, though rather stout whorls. The pile appeared on the sides | 
as faint folds, which are straight at first, then curve, reach the abdomen, and 
finally cross it with a forward inflection. These become very prominent and 
decided before the channel is formed, which finally cuts through the pile. This 
variation, however, is considerable, since in the adult of this specimen the channel 
is only partially developed, the pile being only about half cut through, though 
the specimen is about two and one fourth inches in diameter. There is here a 
close likeness to some of the trias forms, but not to the true planorbe, which the 
young does not resemble closely. They resemble Weehneroceras very closely. 
The same relations were observed in young specimens in Professor Quenstedt’s 
collection, and in the Museum of Comparative Zodlogy. 
It often occurs, also, that after the channel is developed, and the shell is 
quite large, the pile again cross the abdomen, but this is not so frequent as 
has been supposed. They oftener remain separate until old age. 
The original of Amm. angulatus, Sow., which we saw in the British Museum, is 
only a malformed communis." 
1 See also Wright, Lias Amm, p. 478. 
