Geological Societi/. 237 



Museum, the original specimens on which Sowerby founded his 

 Nummular'm elegans (1826, Min. Conch, vol. vi. p. 76). These are 

 partly specimens from that part of the bed " no. 29 " of Prof. 

 Prestwich's section of Alum Bay (Quart, Journ. Geol. Soc. vol. ii. 

 (1846) p. 257, pi. ix. fig, 1) which is known to be the lowest of the 

 Barton series, and partly some in a stone said to be from Emsworth, 

 in Hampshire. The former are the same as those named Niimmu- 

 lites plamdata, var. Prestwicliiana, by llupert Jones in 1852 ; and 

 the latter are N. i^hundata, Lamarck (1804), and probably foreign. 

 Thus N. elegans has priority over Prestwicltiana ; and as this last was 

 determined by De la Harpe to be a variety of N. wemmelensis, Van 

 den Brocck and De la Harpe, this variety should be var. elegans. 

 The Autlior thinks that, on broad zoological principles, N. j)lanulata 

 might still be regarded as the species ; but, in view of the careful 

 differentiation worked out by De la Harpe, he accepts the " specific " 

 standing of " iremmeJensis'^ as useful among Nummiilites ; but '•'Prest- 

 ivichiaaa '' has to give way to " elegans" for the peculiar " Barton " 

 variety. A bibliographical history of the long-misunderstood N. 

 elegans, Sowerby, descriptions of this formand of iV. variolaria (Lam.), 

 notes on N, la'vigata (Brug.), and an account of their range in 

 England completed the paper. 



2. " On the Dentition and Affinities of the Selachian Genus 

 Ptychodus, Agassiz." By A. Smith Woodward, Esq., P.O.S. 



The genus Ptgchodus, owing to the detached condition in which 

 the teeth are usually found, has hitherto been imperfectly under- 

 stood, Agassiz referred it to the Cestraciontida^ on account of a 

 supposed resemblance in the arrangement of the teeth, and Owen's 

 researches on their microscopic structure served to confirm this view. 

 On the other hand, several writers have pointed out characters 

 tending to show affinity between Ptgchodus and lihynchohatas. 



More recently, however, Prof, Cope and the Author had shown 

 that the supposed affinities between Ptychodus and the Cesrracion- 

 tidae were only apparent, and in the present paper additional evi- 

 dence was brought forward. 



The Author proceeded to describe several specimens of P. decur- 

 rens in the British Museum, and in the collection of Mr. H. Wil- 

 lett, of Brighton, one of the latter, especially, containing, what 

 had been previously entirely unknown, the dentition in part, 

 of both jaws. These specimens showed that each jaw contained 

 six or seven longitudinal rows of teeth on each side of the median 

 row, and that the genus must be referred to the true Rays and not 

 to the Cestraciont Sharks, though the precise family to wliich Ptyclio- 

 dus belongs was more difficult to determine. On the whole the 

 writer was disposed to assign it a place either amongst the Mylio- 

 batidte or in their neighbourhood. The microscopic structure of the 

 teeth was shown to be insufficient, by itself, to show their affinities. 



