Vol. 67.] AND Til KIR DISTRIBUTION- IN THE ENGLISH CHALK. ^69 



Ptychodcs polygyrus, var. concentric us (Agassiz). 



This name was given by Agassiz ' to an imperfect tooth from the 

 Upper Cretaceous of Quedlinburg, characterized by a rather high 

 crown, well-marked concentric ridges in the median part, and a 

 wide margin of fine granulation. I have not found any exactly 

 similar teeth in the English Chalk ; but one which very nearly 

 approaches it, from the Holaster-suhglobosus Zone at Bluebell Hill, 

 1 consider to be one of the abnormal forms of Pt. deeurrens 

 (PI. XIX, fig. 18). 



Ptychodtts polygyrus, var. marginalis (Agassiz). (PI. XXI, 

 tigs. 4a-4c.) 



As defined by Agassiz," the teeth of this form have a flattened 

 raised median part, crossed by numerous ridges, which are only 

 sometimes concentric, and bounded by a wide coarsely granulated 

 area. I have only met with one tooth (PI. XXI, fig. 4 a) from 

 Hailing and an associated set of fifteen teeth from Bluebell Hill 

 (zone of Terebraiulina gracilis) which well illustrate the foregoing 

 features ; two teeth belonging to this set are figured with the 

 previous example (tigs. 4 b & 4 c). They tend to connect Pt. poly- 

 gyrus with Pt. mammillaris. 



Teeth of Ptychodus polygyrus are commonest in the zones of Rhyn- 

 chonella cuvieri and Terebraiulina gracilis. 



An associated set of about fifty teeth of this variety from Beachv 

 Head (BAI. P 0141) is marked by parallel grooves with 

 only slightly curved ends; and there are also similar sets 

 in the British Museum from Hailing and Bluebell Hill (36747, 

 47907). I have also sets from Burhara and "Wouldham which agree 

 with those just mentioned (PI. XXII, fig. 2). In some cases, 

 however, the teeth are fairly flat and the grooves on the 

 teeth are finer, more numerous, and often concentric, 

 at least in the principal rows: and I have obtained in- 

 structive examples from Tingey's Pit, Wouldham. In one 

 associated series of a hundred and thirty teeth (B.M. P 10464), 

 which contains six of the small median teeth, I have been able to 

 arrange and identify most of the rows of both jaws. Three teeth 

 from the (hypothetical) upper set are shown (PI. XXII, fig. ]). 



An associated set of seventeen teeth from the zone of Actinoeamcuv 

 quadratus of Northern France described by AT. Leriche 3 is of much 

 interest, as showing a gradation between Pt. polygyrus and 

 Pt. latizsimus. Xearly similar are some teeth from Wantage, pre- 

 served in the Museum of Practical Geology. 



An interesting variety is shown in PI. XXI, fig. 3, which consists 

 of five teeth selected from an associated set containing twenty-two 



i ' Poissons Fossiles ' vol. iii (1833-45) p. 156 & pi. xxv b, fig. 22. 



2 Ibid. p. 157 & pi. xxv. figs. 4-8. 



3 ' Revision de la Faune iehtbvologique des Terrains cretac^s du Nord de la 

 France ' Ann. Soc. Geol. Xord, vol. xxxi (1902) p. <J ( J & pi. ii, fig. 23. 



