392 Geological Society. 



PEOCEEDINGS OF LEARNED SOCIETIES. 



GEOLOGICAL SOCIETY. 



March Sfch, 1911.— Prof. W. W. Watts, Sc.D., M.Sc, E.R.S., 

 President, iu the Chair. 



The following communication was read: — 



' On the Teeth of the Genus Ptijcliodas, and their Distribution 

 in the English Chalk.' By George Edward Diblej^ E.G.S. 



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

 fish genus Ptycliodus, 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 Valleys, West 

 Kent, and the adjoining parts of Surrej^, with reference to the 

 zonal distribution of the genus. 



The Medway Valley affords special facilities for such investi- 

 gations, as there are numerous quarries worked there, from the 

 Micraster cor-anr/uiiium zone down to the Chalk Marl. In addition 

 to material from the above-mentioned localities, material collected 

 from the contents of the chief provincial museums, and also the 

 specimens in the jSTational Collections at the British Museum (Natural 

 History) and the Museum of Practical Geology, Jermyn 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 Ptycliodas 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. Prom 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. j^oly- 

 rjyrus, 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. 



