270 MR. G. E. DIBLEr OX THE TEETH OF PTYCHODUS [May 10. I I, 



teeth from Dunton Green (probably the top of the ffliynchoiiella- 

 cuvieri Zone). These teeth are very flat, and possess a finely 

 granulated margin, which is very broad on the small 

 lateral teeth. 



Ptychodus polygyrus, var. sulcattts (Agassiz). 



The teeth thus named x are essentially intermediate between 

 Pt. polygyrus and Pt. decurrens. Until similar specimens are 

 discovered in an associated set, it is impossible to decide which 

 species they represent. 



Geological Range. 



The typical teeth of Pt. polygyrus are found exclusively iu the 

 zone of Micraster cor-anguinum. The species ranges from the zone 

 of Rhynclionella cuvieri upwards to that of M. cor-anguinum, where 

 it is rare, and afterwards disappears. 



Teeth of the typical form, from the Niobrara Chalk of Kansas 

 (U.S.A.), have also been described under the name of Pt. martini by 

 Prof. Williston.- 



3. Ptychodus latissimus Agassiz. (PJ. XX, figs. 3-5.) 



1839. L. Agassiz, ' Poissons Fossiles ' vol. iii, p. 157, pi. xxva & pi. xxvb, 

 figs. 24-26. 



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

 Transverse ridges very large and few, four or five in the big 

 median teeth, two or three in the small lateral teeth ; slightly 

 curved at the ends. The coarse marginal granulation laterally 

 tends to continue the transverse ridges, and is anteriorly arranged 

 in more or less distinct radiating lines. 



Observations. — This species is well marked, and the teeth 

 referred to it are less variable than those of other species. I have 

 obtained an associated series of thirty-six teeth from the zone of 

 Holaster planus at Bluebell Hill, Burham (PI. XX, fig. 3), and 

 there is another series (artificially arranged on a block of chalk) in 

 the Willett Collection, Brighton Museum. 



Geological range. — Most of the teeth of this species have 

 hitherto been found isolated, and I have only met with them in 

 the zones of Terebratulina gracilis and Holaster planus in Kent 

 and Surrey. 



4. Ptychodus dixoxi, sp. no v. (PI. XX, figs. 1 & 2.) 



1850. Pt. latissimus, Dixon (non Agassiz), ' Geology of Sussex 'p. 362 &pl. xxx, 

 figs. 1 & 2. [Fig. 2 is Pt. polygyrus (Ag.), high zonal form. J 



~% The tooth figured by Dixon as Pt. latissimus differs from other 

 known species ; it does not agree with Agassiz's type, which is 

 shown in fig. 3, and to which Dixon gave the name of Pt. pauci- 

 sulcatus, evidently in error. 



1 Op. cit. p. 156, pi. xxv, fig. 9 & pi. xxv b, fig. 21. 



2 Univ. Geql. Snrv. Kansas, vol. vi (Palaeontology) pt. ii (1900) p. 2^0. 



