552 
foration and the curvature of the first whorl is so uniform that its 
early origin cannot reasonably be attributed to that asa cause. The 
furrow deepens immediately and affects the outline of the fourth 
suture. A slight dorsal lobe appears in the suture of the third 
septum at the sarne time with the annular lobe, and is better 
given in Fig. 33 than in Fig. 34. The. flattening of the dorsum 
is apparent in the second suture, and, so far as I could see after 
repeated observations, my former figure in Amdryology of Cephalo- 
pods was erroneous in placing an annular lobe in this suture. This 
species shows highly accelerated development in all of its charac- 
teristics and this acceleration is obviously genetic and independent 
of the size of the umbilical perforation, which is very large con- 
sidering the fact that it is a Jurassic species. 
I have also examined another less perfect specimen of this species 
having a considerable part of the shell preserved, but the first and 
second apical chambers were lost. ‘lhe external. shell of the um- 
bilical zones had longitudinal ridges as well as external parts of the 
lateral zones and the venter in the paranepionic substage. The 
form of the whorl in section near the ends of the paranepionic sub- 
stage remains about the same, except that the venter becomes 
slightly broader and flatter. The umbilical perforation is not quite 
so large and the gyroceran bend is more abrupt in this specimen, 
but otherwise it is exactly similar to the first described specimen. 
It is in Museum of Boston Society of Natural History. 
CENOCERAS CLAUSUM. 
NAUTILUS CLausus, D’Orb. (Zerr. Jurass., Pl. xxxiii). 
Loc., St. Vigor, near Bayeux, Inf. Oolite. 
Pl. xu, Figs. 12-15. 
This species has a small umbilical perforation. The form and 
general aspect are very similar to those of other compressed shells 
of this genus, but the shell in the paranepionic substage has peculi- 
arly well-marked and broad growth bands with interrupted longitu- 
dinal ridges. ‘The ana- and metanepionic volutions are shown in 
Figs. 13-15 and have a rounded dorsum, the dorsal furrow appears 
in the paranepionic at the gyroceran bend and deepens rapidly as 
the shell grows around the perforation, ‘The amount of involution 
is probably about the same as in Cenoceras granulosus, which it also 
resembles in general aspect as well as in ornamentation. 
