522 
pervforatum as having an apical chamber in which the breadth but 
little exceeds the ventro-dorsal diameter and doubtless at the apex 
itself in the ananepionic substage, as in most Nautiloids, the ventro- 
dorsal diameter exceeds the transverse. 
The coiling is so loose that the umbilical perforation is of very 
large diameter, and the impressed zone, generated only after con- 
tact, is very slight even on the second whorl and does not persist 
after the whorls separate. The dorsum is, however, affected toa 
limited extent on the free whorl in some species as shown in 
Schréder’s admirable figures. These figures give one great satisfac- 
tion, their accuracy, size and detail being full of information. 
The whorls touch during the neanic stage only in some species, 
LEistonioceras ariense, but in others they may continue in contact 
probably throughout the ephebic or a large part of that stage, 
Eston. perforatum and imperfectum. 
_ The lateral angles become rounded in the ephebic stage, but there 
is no tendency to form lateral zones or to flatten the abdomen as in 
Falcilituites. The whorls simply become rounded, depressed ovals 
artd in the paragerontic stage the length of the transverse diame- 
ters decrease. 
The siphuncle is fully described by Schréder in Lstonioceras tm- 
pervfectum, and it is plainly ellipochoanoidal in the ephebic stage. 
What it may be in the young has not been determined. ‘The fun- 
nels are very short and the connecting walls thin and long. The 
position is subventran in the young, tending more towards the 
centre and becoming extracentroventran in some species with the 
advance of age. In the paranepionic substage it may again return 
to a position nearer the venter. 
The species described by Schréder are as follows: 
Lstonioceras perforatum, Schroder; Estonioceras (Lit.) lamellosum, 
sp. His. Drscoceras lamellosum as figured by Angelin et Lindstrom 
seems quite distinct with closer coiled whorls and deeper impressed 
zone, too deep in fact for a species of this genus, whereas Hisin- 
ger’s figure in the Ze¢hea suecica is a true Estonioceras. Never- 
theless Schroder asserts that both figures were made from the same 
“individual.” stonioceras (Lit.) heros, sp. Remelé; avéense, sp. 
Schmidt ; (Lz¢.) cmperfectum, sp. Quenstedt. 
Listonioceras muclauert and Schréder’s Lstonioceras dechent have 
been referred to the genus Falcilituites of Remelé in this paper. 
