136 Geological Society. 



It is possible that septal sections may also furnish the clue to 

 the probable lines along which simplification of the suture pro- 

 ceeds in the retrogressive members of any stock. 



Asymmetry of the septum, and of the suture-line, in ammonites 

 is more common than is usually supposed. It may arise in one of 

 two ways, namely : — 



(1) By the dififerent development of the elements of opposite sides. 



(2) In association with the lateral displacement of the siphuncle. 



Asymmetry of the latter type has been considered as of systematic 

 importance. Nevertheless, while it does arise more frequently 

 in certain genera, as, for instance, in Psiloceras and Hoplites, 

 it occurs not uncommonly in many other unkeeled ammonites. 



May 2ad, 1917.— Dr. Alfred Harker, F.R.S., President, 

 in the Chair. 



The following communication was read:— 



1. ' Supplementary Notes on AcUsina De Koninck, and AcU- 

 soides Donald, with Descriptions of New Species.' By Jane 

 Longstaff (nee Donald), F.L.S. 



Since the publication of a paper by the Greological Society on 

 Aclisina in 1898, a much larger amount of material has come to 

 hand, which has not only added to the knowledge of the species 

 there described, but has also led to the discovery of six others new 

 to science. The diagnoses of these are now given, and a species 

 named by Mr. H. Bolton Loxoiiemcv ashfonensis is referred to 

 this genus, as several specimens show the characteristic lines of 

 growth. 



The total number of species of Aclisina is now brought up to 

 twenty-two, and there are besides several varieties. The genus is 

 best represented in Scotland, where the specimens are generally 

 remarkabl}' well preserved, no less than thirteen having the proto- 

 conch intact, di'awings of which show its somewhat irregular 

 character. A table is appended giving — so far as known — the 

 range and localities in the British Isles and Belgium. A small 

 variety of Aclisina pidchra De Koninck appears to have continued 

 for the greatest length of time, commencing in the Calciferous 

 Sandstone Series, existing throughout the Lower and Upper 

 Limestone Series and on into the Millstone Grit of Scotland. 



Additional observations are also made on Aclisoides striatula 

 (De Koninck), showing its variation in size and ornamentation, 

 as well as its range throughout the Lower and Upper Carboniferous 

 Series of Scotland, its occurrence at Settle and Poolvash, and at 

 Tournai as well as Vise. New drawings are given of De Koninck's 

 type-forms. One of these, as also a Scottish examj^le, has the 

 characteristic sinus preserved. The holotvpes of the first described 

 species were not originally selected: that omission is now rectified, 

 with references to the collections in which they are de^DOsited. 



