521 
and 21, have saddles on the venter with perhaps very slight ventral 
lobes on either side of this, saddles at the lateral angles and faint 
dorsal saddles. Schréder also describes the same substages in 
Estonioceras perforatum and the sutures of the first whorl, but his 
descriptive nomenclature is not clear and the text is consequently 
not perfectly intelligible. Apparently the first whorl has broad 
ventral saddles and nearly straight dorsal sutures with sometimes 
very slight dorsal saddles. He states very distinctly, however, when 
the whorls touch, at or near the end of the first whorl, that a decided 
change takes place, the dorsal sutures acquiring well-marked dorsal 
lobes. He also clearly states that in the uncoiled volutions this 
dorsal lobe, although it persists, loses in height and breadth. 
In his descriptions of Lztuctes Mucllaurét, of Dewitz, he makes 
these statements clearer by saying that the dorsal and ventral sad- 
dles possessed by the adult of this species are similar to the sutures 
of the young species of Estoniceras perforatum and imperfectum, 
which have sutures with ventral and dorsal saddles as figured by 
him only on the venter of Zstoniceras imperfectum. There is also 
a shallow lobe developed in the middle of the dorsai saddle in 
Estoniceras perforatum, which persists even in the gerontic stage. 
After the lateral angles disappear in the ephebic stage, the saddles 
and lobes are less prominent and become almost straight in the 
more rounded volutions of the gerontic stage, but the approximate 
return of the same outlines as are found in the paranepionic substage 
is plainly visible, Figs. 17 and 18, Pl. vil. 
The lines of growth show a broad hyponomic sinus and lateral 
crests which increase in prominence towards the dorsum, but 
directly in the centre of that side there is a crest and on either side 
of this shallow dorsal sinuses. ‘There is, however, a dorsal sinus 
which persists in the gerontic stage in the lines of growth of a 
specimen figured by Schréder of Zstontoceras heros.* 
The aperture is wide in the ephebic stage, but evidently contracts 
with the whorl in extreme age, as shown in Schréder’s figure of 
Estonioceras imperfectum, Pi. iii, Fig. 2, b, and in the figures of 
Estonioceras biangulatum in this paper, Pl. vii, and sometimes the 
ventro-dorsal diameter may become longer than the transverse in 
the paragerontic substage. This is, in part, a return to the early 
proportions, since Schréder describes the apex of his “’s/onzoceras 
* Op. cit. Pl. v. 
PROC. AMER. PHILOS. SOC. XXxII. 148 3N. PRINTED JULY 10, 1894. 
