122 GENESIS OF THE ARIETIDA. 
than usual, and the involution slightly increased, a modification. which is also 
sometimes present, though less marked, in erugatus. 
Wihner found what he claims to be Psi. planorbe at Pfonsjoch in the Pla- 
norbis bed. ‘These were small specimens, measuring 15-40 mm. in diameter, and 
one of them is said to be similar to Hagenowi In the same work he figures 
the following discoidal shells of the smooth subseries: Psi/. polycyclum and cali- 
phyllum, Plate XV., and Psil. pleurolissum.2,Neumayr, in the Unterster Lias,? 
gives Psil. planorboiles, a more involute, smooth species of this series. Planorboides 
appears to lead into two much more involute and compressed species figured by 
Wiihner in the same work, Psi. Atanatense and mesogenost Both of these are 
devoid of true pils, and possess only senile fold-like pilations.® 
SECOND SUBSERIES. 
Psiloceras longipontinum, WAuner. 
Amm. longipontinus, Opp., Pal. Mittheil., p. 129, pl. xli. 
Psil. longipontinus, WAuNER, Unt. Lias, Mojsis. et. Neum., Beitr., IV. p. 196. 
4igoc. Clausi, Nnum., Unterst. Lias, Abh. k. k, geol. Reichsans., VII. pl. iii. 
The original of this species in the Museum of Stuttgardt has considerable like- 
ness to Psi/. planorbe, var. plicatum. Oppel seems to have considered it one of the 
schlotheimian series.’ The open umbilicus, straight folds in place of true pile, 
keelless abdomen, and helmet-shaped form of whorl, show it to be a member of 
the psiloceran series. The sutures,’ as figured by Oppel, exhibit the strong psilo- 
ceran affinities of the species. In his specimen the last whorl has become smooth 
on one side, and the pile nearly obsolete on the other, thus indicating the 
approach of senility, though the shell is but 95 mm. in diameter. The pile begin 
to obsolesce posterior to the last septum. The living chamber is nearly one volu- 
tion in length, though still incomplete. An empty cast in the Semur Museum 
from Ruffy undoubtedly belongs to this species; it is 155 mmi.in diameter, and the 
last whorl is smooth, showing its great age. There are specimens in the collec- 
tion at Munich labelled Amm. Roberti, Hauer, locality Filder, and Amm. Oceduensis, 
1 Unter. Lias, Mojsis. et Neum., Beitr., IV. p. 186. me Meats (SEES oj ze oe ah 
8 Abhandl. geol. Reichsans., VII. pl. iv. 4 Op. cit., III. pl. xxvi. 
> We have figured only the most involute of this smooth series on Summ. Pl. xi. fig. 13. 
° The closeness of the parallellism between some of the forms of Psiloceras and some species 
of the Lytoceratide is such as will be likely to cause considerable confusion unless great care is taken 
in studying the species. Comparison of such forms as Amm. Petersi, Hauer, Ceph. nordost. Alpen, 
pl. xxi. fig. 1-3, Lyt. Petersi, Herb., Széklerland, pl. xx., Lyt. 2 Driani, sp. Dumort., Etudes Pal. du Basin du 
Rhone, and Lytoc. (Amm.) Roberti, Hauer, Capric. oesterr. Alpen, pl. iii., will show that without close 
study of the sutures and young no separation can be made with certainty. In fact, in identifying Driani 
in the absence of figures of the sutures as a form of Lytoceras, we have been led by the geological position 
and size, which accord better with Lytoceras than with a species of Psiloceras. It is possible that in doing 
this we are illustrating these remarks in a forcible manner. 
See also in this connection the forms of Rhacophyllites and Phylloceras figured by Canavari in his 
‘Fauna des unteren Lias von Spezia,’’ pl. ii. 
* The sutures figured by Portlock, as well as the form of the section (b) of his Amm. Sampsoni (fig. 13 c, 
not fig. 13 a), suggest longipontinus, and may indicate the presence of this form, or transitional varieties, in 
the English basin. 
