268 ME. G. E. DIBLEY ON THE TEETH OF PTYCHODVS [May K)J1, 



Upper Greensand. I have not yet obtained a specimen from the 

 zone of Ammonites rhotomagensis? 



It is interesting to note that Pt. decurrens is the only species from 

 the English Chalk of which the teeth have been found in their 

 natural arrangement. 



2. Ptychodtts polygteus Agassi/,. (PI. XX, figs. 1 & 2 ; PI. XXI, 



figs. 1 & 2.) 



1839. L. Agassiz, ' Poissons Fossiles ' vol. iii, p. 156, pi. xxv, figs. 10-11 & 

 pi. xxv b, fig. 23. 



Specific characters. — Central part of crown of tooth flattened. 

 Transverse ridges relatively large, curving round more or less at the 

 lateral ends, sometimes forming gyrations ; marginal granulated 

 area usually of considerable extent, coarsely marked, never impressed 

 with radiating grooves. 



Variations. — The teeth referred to this species are remarkably 

 varied, and Agassiz divided them into four groups to which he gave 

 provisional names. My studies of numerous associated sets of 

 teeth prove that there are gradations between the extreme types, 

 and I therefore continue to regard the different forms merely as 

 varieties. 



In the typical Pt. polygyrus, as defined by Agassiz, the principal 

 teeth are much flattened, the grooving is very coarse, with frequent 

 gyrations, and the marginal granulation is only wide in some of the 

 large teeth of the lower median row. One of these lower median 

 teeth is figured by Agassiz 2 under the name of Pt. latissimus. The 

 finest known specimen, showing the greater part of the dentition 

 of both jaws, was obtained by Mr. W. Murton Holmes from the 

 zone of Micraster cor-anguinum at Banstead (Surrey), and is now 

 in the British Museum (P 10771). It comprises about 150 scattered 

 teeth, which I have been able to arrange approximately in their 

 natural order. Besides those of the median series, there are also 

 teeth of five or six paired rows in each jaw, and a selection is 

 shown in PI. XXI, figs. 1 & 2. 



An associated set of about twenty similar teeth has been discovered 

 by Mr. C. Gosling, in the same zone of the Chalk, at Croydon ; and 

 it may be added that there is another set in the British Museum 

 (P 4551) from the Chalk of Normandy. 



The form just described appears to be characteristic of the 

 Micraster cor-anguinum Zone. 



1 [After this paper bad been read, Mr. E. T. Kewton kindly told me that there 

 was a tooth of Ftychodus among a collection of fossils from the Potton Beds of 

 the Lower Greensand, at the Museum of Practical Geology, Jermyn Street. I 

 have now seen this specimen, which is iron -stained just as are the other 

 fossils from this horizon : it is undoubtedly a tooth of Pt. decurrens var. de- 

 pressus. It was unrecorded by Mr. Keeping in ' The Fossils & Palasontological 

 Affinities of the Neocomian Deposits of Upware & Brickhill' Cambridge, 1883. 

 I consider it to be a derived fossil.— G. E. B., March 22nd, 1911.] 



2 Op. cit. pi. xxv a, fig. 8. 



