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Jura, but the observations ou 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 PL 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, n. s. 



This has affinities with excavatus, but the large fine young speci- 

 men, Figs. 6-1 1, 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. excavatilm 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 Digonioceras 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. 



