ra 
639 
PLATE XIII. 
Figs. 1 and 2. Cymatoceras ( Naut.) radiatum, showing the reverse of Fig. 30, 
Pl. 12, enlarged to 2 diameters. The beginning of the dorsal furrow is indi- 
cated by the shaded area reaching from near the first septum to the edge of the 
lower or fifth septum. 
Fig. 2, side view of same, showing that the dorsal furrow began on the dorsum 
of the metanepionic substage between the first and second septa and in advance 
of the gyroceran bend. 
Fig. 3. ELztrephoceras (sp. ?); Loc., France; Duval Coll., Mus. Comp. Zodl. ; 
Cretaceous; natural size; showing the dorsal furrow in the meta- and paranepi- 
onic substages and section of the ananeanic stage below. 
Figs. 4-8. Lutrephoceras De Kayt, Dakotah, Cretaceous, preparations by 
Henry Brooks. Fig. 4, enlarged 6 diameters, and Fig. 5 same, enlarged 4 diam- 
eters, view of ana-, meta- and part of paranepionic substages. The citatrix is a 
double depression and the ananepionic outline is given just beyond this. There is 
a plate of the nacreous layer ventrad of this and partly covering it, with a shaded 
area. ‘This spot is evidently the apex of the caecum seen through the nacre. The 
two first substages are very short and smooth, but the dorsal furrow is present 
although exceedingly shallow before the bending begins in the later metanepionic. 
The umbilical perforation is present, as shown in Fig. 4, but is very small and 
elongated, comma-like in shape. Fig. 6, enlarged 4 diameters, shows the perfo- 
ration in an older stage, but it is not correctly given. It is exposed by shaving 
off the angle of the last septum and the perforation is consequently actually the 
reverse of what it is in the centre of the umbilicus. This preparation, however, 
does show accurately the contact of the paranepionic dorsum with the dorsal side 
of the ananepionic substage and how close the coiling is. Figs. 7 and 8, side and 
front view enlarged 2 diameters of the meta- and paranepionic substages, the 
ornamentation becoming less in the latter which is terminated by a permanent 
constriction in this specimen, and also the anephebic substage in which the longi- 
tudinal ridges disappear and bands of growth assume the fine unbroken outlines 
of the adult. Fig. 7 is erroneous in making too great difference between the 
ventral lines of growth in the young parts of the whorl. The hyponomic sinus 
really appears about the middle of the paranepionic substage. 
Figs. 9-12. Eutrephoceras Faxoense, Faxoe, Denmark; Krantz Coll., Mus. 
Comp. Zoélogy; Cretaceous. Fig. 9, front view, natural size, showing the cast of 
the umbilicus continuous with the very small umbilical perforation of the young. 
Fig. 10, side view of the same specimen. Fig. 11, young with first septum delin- 
ealed. Fig. 12, front view of same, showing the aspect of the apex and the um- 
bilical perforation, the dorsal furrow apparently beginning as in Lutrephoceras 
De Kayz. Both of these are enlarged 4 diameters. 
Figs. 13-16. Lutrephoceras (Naut.) imperialis (sp. Sow.); Isle of Sheppy 
and Isle of Wight; Mus. Comp. Zoélogy; Tertiary. Fig. 13, front view of inner 
whorls enlarged 2 diameters. Fig. 14, fragment of nepionic or neanic stages, 
showing the minute umbilical perforation, the absolutely subdorsan position of 
siphuncle in these early substages. This specimen has a double first septum. 
Fig. 15, front of specimen from Isle of Sheppy, showing similar position of 
