617 
EXPLANATION OF PLATES. 
RA mE Te 
- Illustrations of nepionic stage and ananeanic substage in (Vaztilus pompilius 
from preparations made by Henry Brooks and drawn under his direction. They 
are all enlarged to show the details uf the surface ornamentation and changes of 
form. 
Fig. 1. The paranepionic aperture and earlier nepionic substages seen from 
the front, this preparation having been obtained by breaking down the full- 
grown shell. The septum which appears here necessarily belongs to a later time 
and shows the position of the siphuncle and its large size in the floor of a living 
chamber older than is represented in this figure, but in the same substage. The 
actual living chamber of this age was therefore deeper than is represented here at 
the beginning of the paranepionic substage. The subtriangular outline of the sec- 
tion of the shell is supposed to represent and probably does approximately repre- 
sent the aperture. It is noticeable also that the dorsal furrow is well developed, 
although the shell has not yet completed the gyroceran curve. 
Fig. 2. Side view of the same showing the apex. The ananepionic substage 
is not distinctly visible, but the constrictions showing apertures of the metane- 
pionic substages are delineated.* This figure is especially intended to exhibit 
the changes that take place in the ornamentation of the shell. 
Fig. 3. The ananepionic and metanepionic substages in another preparation 
seen from the front. The ananepionic is the elongated disk of the apex and scar in 
the centre of this. The metanepionic includes the shell outside of this to the 
outer constriction. 
The details in this have not been completely drawn, but the transverse lines of 
growth are shown upon the shaded side of the drawing. 
Fig. 4. Same from the ventral side, showing especially the latter part of the 
metanepionic substage and the deep constriction that in some specimens marks 
the termination of this substage. The lines of growth show no trace of a hypo- 
nomic sinus in this or any other preparation at this age. 
Fig. 5. The same substages of the nepionic stage seen from the front in another 
preparation. The details of the transverse striation are not fully given in this 
drawing and this makes the ananepionic substage appear more gibbous than it 
really is. This impression is corrected by Fig. 6. 
Fig. 6. View of the same from the venter, very carefully finished in all its 
details. The limits of the ananepionic substage and the aperture of the first part 
of the metanepionic substage are more plainly marked in this specimen than is 
usual. 
* These drawings and others in this paper will appear to most observers to be upside 
down. They are really right side up and the conventional mode of representing these 
shells followed hitherto in all works is unnatural. It is full time that these forms should 
be pictured, as are all others in scientific and popular works, as they stand in nature. 
The greatest objection to this is the inconvenience of comparison with illustrations 
hitherto published, but this cannot be avoided, and must be endured for the sake of 
progress. 
PROC. AMER. PHILOS. SOC. XXxII. 148. 3 zZ. PRINTED AUGUST 6, 1894. 
