549 
Jura, but the observations on the young given in this paper show 
- that these forms are not so closely related as I then supposed. 
The species differ from any species of Endolobus in the form of 
the young and in having the annular lobe and dorsal furrow devel- 
oped earlier and in not having any large nodular tubercles. The 
form of the whorl in section is, however, similiar in adults of Digo- 
nioceras and also the aperture. The umbilical perforation is larger 
than usual in other allied genera of the same period and the invo- 
lution is apt to be less in the older substages, leaving the umbilici 
open. 
D’Orbigny’s figure copied on PI. xi, Figs. 13, 14, shows that a 
dorsal furrow was present in the paranepionic substage* and there 
is a similar furrow at the same age in Digonioceras rotundum, the 
type of this genus. 
DIGONIOCERAS, sp. (?) 
The species from Balingen, Middle Lias, Figs. 19-21, Pl. xi, 
shows a form similar to Digonioceras excavatum. The metanepi- 
onic outline in this has no dorsal furrow, as shown in the corrected 
section, Fig. 21, but the suture has an annular lobe. The dorsal 
furrow begins on the dorsum at the second septum of this fragment, 
which was probably the fourth or fifth of the complete shell. 
DIGONIOCERAS ROTUNDUM, DN. Ss. 
This has affinities with excavatus, but the large fine young speci- 
men, Figs. 6-11, Pl. xii, show that the shell was specifically distinct. 
It is obviously from the Oolite, but the locality is not known. 
Figs. 6 and 7 show the neanic stage and Fig. 8 the nepionic. The 
involution is not greater than it is in D. excavatum and the form 
is very similar in the paranepionic. ‘The whorl, however, is really 
nephritic in the ananeanic substage, and has already assumed an 
outline in section quite distinct from that of Dzgontoceras exca- 
vatum. This last-named species retains throughout life, if cor- 
rectly figured by D’Orbigny, the same form as the paranepionic 
volution of D. rotundum shown in Fig. 8. The outer whorl in 
Fig. 7 should be a little broader in proportion and more com- 
pletely nephritic. There is a slight trace or linear depression 
near the median line on the abdomen, but this may be an individual 
character, and not important to the diagnosis of the species. 
* This species, at first referred to the Lias, was subsequently in this author’s Prodrome 
placed in the Inferior Oolite. 
