FIRST, OR PSILOCERAN BRANCH. 121 
First AND SECOND SUBSERIES. 
Psiloceras planorbe, Hyarr. 
Var. leve. 
Plate I. Fig. 1-4. Summ. Pl. XI. Fig. 13 Pl. XII. Fig. 1. 
Amm. planorbis, Sow., Min. Conch., V. p. 69, pl. ececxlviii. 
AEgoc. planorbis, Wrigur, Lias Amm., p. 308, pl. xiv. fig. 1-4. 
Amm. psilonotus levis, QuENStT., Die Ceph., p. 73, pl. iii. fig. 13; Amm. Schwab. Jura, pl. i. fig. 1-7. 
Amm. Sampsoni, Porty., Rep. Geol. Londonderry, ete., p. 188, pl. xxix. a, fig. 13. 
Psil. planorbe, Hyarr, Bull. Mus. Comp. Zool., I. No. 5, p. 73. 
Localities. — Whitby, Watchet, Montloy, Semur, Rudern, Nellingen, Balingen, Neuffen.1 
This remarkable form is a somewhat flattened discoidal and perfectly smooth 
shell in its typical adult form. The young are often plicated. 
Var. plicatum. 
Plate I, Fig. 5,6. Summ. Pl. XI, Fig. 2. 
Amm. psilonotus plicatus, QuensT., Amm. Schwab. Jura, pl. i. fig. 1-14 (mot fig. 8, 18). 
This shell differs from variety /eve merely in having immature pile or folds in 
the neologic and ephebolic stages. There is therefore the most gradual and 
hardly perceptible gradation from the preceding variety to this form. The septa 
of both are exceedingly variable. The marginal digitations may be either very 
shallow, as in the Arietidee generally, or they may be foliaceous and complicated, as 
in the radical series. The lobes and saddles may also vary exceedingly in size 
and proportions; some species have deep and narrow saddles with long broad 
lobes, as in the radical series, while others, more like the typical Arietidx, have 
shallower, broader saddles, and shorter, more pointed lobes. In the collection at 
Semur there are forms from Saulieu identical with the South German, which 
when compared with raricostatum and Johnston’, show closer approximations than 
any specimens seen elsewhere. 
The Bristol collection contains undistorted specimens of this species from 
Cotham, and in Dr. Wright’s collection from Whitby the plhcatus variety is labelled 
Am. erugatus, Bean. The connection with the flattened Watchet specimens of 
planorbis, Sow., can be clearly made out by the large tablet in the British Museum, 
containing about one hundred and fifty specimens. Of these, perhaps ninety 
exhibit folds like those of plicutus and erugatus. The largest on this slab is from 
60 to 80.5 mm. in diameter. These large specimens are not equivalent to Cad. 
Johnston’, as Oppel supposed, but to piicatus. Hrugatus seems to be a dwarfed form 
with the folds often developed very strongly in the young,’ and the shell has fine 
stris of growth, as in Agas. striaries, Plate IX. Fig. 14, 15. In the Museum of 
Comparative Zodlogy the series is complete from /eve to var. plicatum, as figured 
by Quenstedt in “ Der Jura,” and in another direction to the var. of planorbe from 
Semur? This is a slightly plicated form, having the sides of the whorls broader 
1 These localities also include var. plicatum. 8 See Pl. i. fig. 5, 6. 
2 Amm. erugatus Bean has only the young plicated, resembling in this respect var. eve. It is however 
always a small form or dwarf. 
16 
