FIRST, OR PSILOCERAN BRANCH. 121 



First and Second Subseries. 



Psiloceras planorbe, Hyatt. 

 Var. leve. 



Plate I. Fig. 1-4. Sumin. PI. XI. Fig. 1; PI. XII. Fig. 1. 



Amm. planorbis, Sow., Min. Conch., V. p. 69, pi. ceccxlviii. 



JEgoc. planorbis, Wright, Lias Amm., p. 308, pi. xiv. fig. 1-4. 



Amm. psilonotus levis, Quenst., Die Ceph., p. 73, pi. iii. fig. 13; Amm. Schwab. Jura, pi. i. fig. 1-7. 



Amm. Sampsoni, Porti,., Rep. Geol. Londonderry, etc., p. 138, pi. xxix. A, fig. 13. 



Psil. planorbe, Hyatt, Bull. Mus. Comp. Zobl., 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. 



Vak. plicatum. 



Plate I. Fig. 5, 6. Summ. PI. XI. Fig. 2. 



Amm. psilonotus plicatus, Quenst., Amm. Schwab. Jura, pi. i. fig. 1-14 (not fig. 8, 13). 



This shell differs from variety leve merely in having immature pilas 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 Arietidse 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 Arietidas, 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 raricostaium and Johnsioni, 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 plicatus variety is labelled 

 Amm. 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 plicatus and erugatus. The largest on this slab is from 

 60 to 80.5 mm. in diameter. These large specimens are not equivalent to Cat. 

 Johnstoni, as Oppel supposed, but to plicatus. Erugatus seems to be a dwarfed form 

 with the folds often developed very strongly in the young, 2 and the shell has fine 

 stride of growth, as in Agas. striaries, Plate IX. Fig. 14, 15. Tn the Museum of 

 Comparative Zoology the series is complete from leve to var. plicatum, as figured 

 by Quensteclt in "Der Jura," and in another direction to the var. of planorbe from 

 Semur. 3 This is a slightly plicated form, having the sides of the whorls broader 



1 These localities also include var. plicatum. s See PL i. fig. 5, 6. 



2 Amm. erugatus Bean has only the young plicated, resembling in this respect var. leve. It is however 

 always a small form or dwarf. 



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