Reports and Proceedings — Miner alogical Society. 237 



(5) The Foranriniferal Limestones of Cyrenaica. By David Paterson 

 MacDonalcl, M.A., B.Sc. 

 The microscopic examination of the limestones of Cyrenaica shows 

 that they are all mainly composed of organic material, and are 

 remarkably free from detrital material. The rocks were laid down 

 under conditions of quiet sedimentation. Some of them have been 

 partly dolomitized. The chalky limestones near the base of the series 

 have been deposited at the greatest depth. The commonest organisms 

 present are Foraminifera, Echinoderms including holothurian plates, 

 Mollusca, and calcareous Algae, which in some specimens form more 

 than half of the whole rock. Some of the limestones are oolitic. 



2. "On the Teeth of the genus Ptychodus, and their Distribution 

 in the English Chalk." By George Edward Dibley, F.G.S. 



This paper is an attempt to define the species of the fossil fish 

 genus Ptychodus, and gives the result of the investigations pursued by 

 the writer during the past twenty years among the extensive Chalk 

 quarries in the Thames and Medway Yalleys, West Kent, and the 

 adjoining parts of Surrey, with reference to the zonal distribution, of 

 the genus. 



The Medway Valley affords special facilities for such investigations, 

 as there are numerous quarries worked there, from the Hicraster cor- 

 anguinum zone down to the Chalk Marl. In addition to material 

 from the above-mentioned localities, material collected from the con- 

 tents of the chief provincial museums, and also the specimens in the 

 National Collections at the British Museum (Natural History) and 

 the Museum of Practical Geology, Jerniyn Street, have been studied 

 in detail by the author, who has obtained no less than fifty associated 

 sets of teeth from various localities. 



Hitherto, our information as regards Ptychodus has been derived 

 from associated sets of Pt. decurrens in place and isolated teeth of this 

 and other species. The variation in teeth of one individual is often 

 so marked, that when found separately they have given rise to the 

 formation of new species. From evidence now brought together by 

 the author for the first time, it can be proved that these teeth belong 

 to already known species, and merely represent a phase in variation 

 in the development of certain teeth of one species. 



Special attention has been given to the extreme variation in 

 Pt. decurrens, as well as in the equally variable species Pt. poly- 

 yyrus, and one new species has been added. 



A special feature is, that throughout the species a series of teeth 

 extending from the centre to the exterior of the palate is figured, 

 which also for the first time enables the student to form some idea of 

 the variation exhibited by the separate rows, even in the teeth of the 

 same individual, and indicates the care necessary in identifying species 

 when dealing with solitary teeth. 



III. MlNERALOGICAL SOCIETY. 



March 21, 1911.— Prof. W. J. Lewis, F.R.S., President, in the Chair. 



Professor W. J. Lewis : On Mr. Solly's observation of "Wiltshireite 



in 1903. Wiltshireite is identical with the mineral which Mr. Solly 



