[22 GENESIS OF THE AEIETIlLE. 



than usual, and the involution slightly increased, a modification which is also 

 sometimes present, though less marked, in erugatus. 



Wahner found what lie claims to be Psil. 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. 1 In the same work he figures 

 the following discoidal shells of the smooth subseries : Psil. polycyclum and eali- 

 phgllum, Plate XV., and Psil. pleurolissum? Neumayr, in the Unterster Lias, 3 

 o-ives Psil. planorboides, a more involute, smooth species of this series. Planorboides 

 appears to lead into two much more involute and compressed species figured by 

 Wahner in the same work, Psil. Atanatense and mesogenos* Both of these are 

 devoid of true pilaa, and possess only senile fold-like pilations. 8 



Second Subseries. 

 Psiloceras longipontinum, Wahner. 



Amm. longipontinus, Opp., Pal. Mittheil., p. 129, pi. xli. 



Psil. longipontinus, Wahner, Unt. Lias, Jlojsis. et. Neum., Beitr., IV. p. 196. 



Mgoc. Clausi, Neum., Unterst. Lias, Abh. k. k. geol. Reichsans., VII. pi. iii. 



The original of this species in the Museum of Stuttgardt has considerable like- 

 ness to Psil. planorbe, var. pUcatum. Oppel seems to have considered it one of the 

 schlotheimian series. 6 The open umbilicus, straight folds in place of true pilee, 

 keelless abdomen, and helmet-shaped form of whorl, show it to be a member of 

 the psiloceran series. The sutures. 7 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 pilae nearly obsolete on the other, thus indicating the 

 approach of senility, though the shell is but 95 mm. in diameter. The pilaa 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 mm. in diameter, and the 

 last whorl is smooth, showing its great age. There are specimens in the collec- 

 tion at Munich labelled Amm. Eoberti, Hauer, locality Filder, and Amm. Oeduensis, 



1 Unter. Lias, Mojsis. et Neum., Beitr., IV. p. 130. 2 Ibid., III. pi. xxvi. 



3 Abhandl. geol. Reichsans., VII. pi. iv. 4 Op. cit., III. pi. xxvi. 



6 We have figured only the most involute of this smooth series on Summ. PI. xi. fig. 13. 



6 The closeness of the parallellism between some of the forms of Psiloceras and some species 

 of the Lytoceratidse 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. norddst. Alpen, 

 pi. xxi. fig. 1-3, Lyt. Petersi, Herb., Szeklerland, pi. xx., Lyl. ? Driani, sp. Dumort.. Etudes Pal. du Basin du 

 Rhone, and Lytoc. (Amm.) Roberti, Hauer, Capric. oesterr. Alpen, pi. 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 Cauavari in his 

 " Fauna des unteren Lias von Spezia," pi. 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. 



