524 
ventro-dorsal .30 mm. at the termination of the same, the fragment 
of the living chamber also devoid of shell was transversely about 
40 mm., while the dorso-ventral had increased to 32 mm. This 
and the point of view of Fig. 18 gives the aspect of a more con- 
siderable diminution in the lateral diameters than actually took 
place. The sectional view of this end shows true proportions in 
Fig. 19, Pl. vii. 
The section, Fig. 15, gives the neanic whorl at the inner break in 
the side view, Fig. 13, and this shows how very small and slight the 
zone of involution is in this species. It broadens slightly with age, 
but immediately disappears in the free part of the whorl, as it does 
also in most species of this genus. 
The lines of growth are given on the dorsum of the metagerontic 
substage in Fig. 17, and these do not differ materially from those 
of the nepionic stage in Lstonioceras perforatum. ‘The lines of 
growth could not be observed on the dorsum of the earlier stages 
of growth, but it is probable that in the stages in which the whorls 
are in contact that the dorsal crest is narrow and occupies the area 
of the impressed zone. 
The sutures in the paranepionic substage have ventral saddles. 
with very slight ventral lobes on either side, saddles at the lateral 
angles and apparently lateral lobes. ‘Three sutures of this substage 
were followed on the dorsum to the centre and no central inflec- 
tions could be seen. This was somewhere about the sixth or 
eighth septum, as nearly as could be ascertained, and the suture 
formed a very shallow lobe across the dorsum, but this would 
ordinarily be described as straight. ‘These, in other words, are 
closely similar and of about the same age as the complete dorsal 
sutures of Estontoceras imperfectum given in Fig. 21, Pl. vii, and 
probably about the same age as the dorsal sutures of Estonioceras 
perforatum given in Fig. 12, Pl. vii. In the ephebic stage broad 
lobes appear on the venter, reaching to the saddles at the lateral 
angles. The septum, Fig. 15a, Pl. vii, given to show the contact 
furrow, also shows that a faint narrow dorsal lobe coextensive 
with this furrow is produced by contact. In the paragerontic 
substage, as shown on the last three sutures of Fig. 18, the broad 
ventral lobe is replaced by faint saddles with very faintly marked 
lobes on either side and the saddles of the lateral angles in conse- 
quence of the rounding of these angles have become lateral saddles. 
The lateral lobes appear only very faintly or are absent on the 
